I think this story must be "The Monkeys Thought 'Twas All In Fun" (1979), by Orson Scott Card, which matches your description closely.
The story is of a planet-sized smooth black object/creature appearing (close to earth?) and astronauts being sent to explore and find out what it is. They land and find they can push(sink) into the surface, far enough to pass through the outer skin and find themselves in an earth-like paradise with air, mountains, trees, rivers etc.
... "This surface material sucks up all all light. Even from my headlamp. Hard and smooth as steel, though. I have to keep shining my light on my hands to see where they are." Silence for a few moments. "Can't tell if I'm scratching the thing or not. Am I getting a sample?"
...
Agnes frankly admitted that there wasn't anything she could possibly do. While she was admitting it she clambered out of the skipship and launched herself toward the surface.
...
But as she touched the surface, it yielded. Not with the springiness of rubber, which would have forced her hand back out, but with the thick resistance of almost-hard cement, so that she found her hand completely immersed in the surface of the planet. She shone her headlamp on it–the smooth surface of the planet was unbroken, not even dented, except that her hand was in it up to the wrist.
...
Agnes and Danny stood on a mountaintop, or what had seemed to be a mountaintop from the skipship. They had reached it after only a few hours' walk, much of it sped by shaddling, and learned that what seemed to be a high mountain was only a few hundred meters high, maybe even half a kilometer. It was rugged enough, though, and the climb, even shaddled, had not been easy.
"Artificial," Danny said, touching the wall with his hand. The wall ran from the top of the mountain up to the ceiling, where instead of a sun the whole ceiling glowed with light and warmth, as thorough as sunlight, yet diffused so that they could look at it for a few seconds without being blinded.
"I thought we concluded this place was artificial from the beginning," Agnes said.
"But what's it for?" Danny asked, letting his frustration at two days of exploration come to the surface. "Bare dirt, rich enough but with not a damn thing growing. Clean, drinkable water. Rain twice a day for twenty minutes, a gentle sprinkle that wets everything but creates almost no runoff. Sunlight constantly. A perfect environment. But for what! What lives here?"
The astronauts find that they can also push through the floor of this first paradise into yet another 'bubble' or pod of this earth-like environment and find many more similar 'pods' in all directions...
He went to the surface and told Agnes what he had found. They swam to shore, put their suits back on, and shaddled down into the water. The lake floor opened, engulfed them, and then floated them out the bottom–into the sky directly over the skipship, where it still rested on the surface of the lake. They shaddled safely down.
"This place is explorable," Agnes told Roj and Roz, "and it's simple. It's like a huge balloon, with other balloons inside and more and more of them, layer after layer. It's designed for somebody to live here, so when you're standing on the soil you don't sink through. To get down, you have to go through the lake."
...and ultimately decide to colonise (maybe with unwanted earth prisoners at first) the expansive spaces of this new world/planet/creature thing.
Eleven years and eight hundred billion dollars later, IBM-ITT's ships were in the sky, filling with colonists. GM-Texaco's ships were still under construction, and five other consortiums would soon be in the business. More than a hundred million people had signed up for seats on the ships. The seats were free–all it took was a deed made out to the corporation for all the property a person owned, in return for which he would receive a large plot of ground in the Balloon. Whole villages had signed up. Whole nations were being decimated by emigration. The world had grown so full that there had been no place to run away to. Now there was a new promised land. And at the age of forty-two, Agnes brought her ship forward to part the waters.
During the story there is a narrative of an 'elder being' teaching its young about their own purpose (I always imagined 'story-time' around a campfire)...
The object is a group of alien creatures called "Hectors", and the story is structured as a sequence of sections which alternate between Hector and Agnes. The Hector sections consist of the creatures telling stories to themselves.
...and the story ends with the 'elder' exploding and all the child environmental 'pods' and their containing colony of humans being flung to the corners of the universe to populate it with the life that they now contained.
Not exactly – the object explodes killing everyone inside, but an essence of the object survives as dust launched into space. It benefited from being colonized by humans as it learnt their stories.
Every wall split into two thinner walls, and every cell detached from every other cell. For a moment they hung there in space, separated by only a few centimeters, each from the other; but all still were linked to each other through the center, where vast forces played, forces stronger than any in the solar system except the fires of the sun, which had been the source of all the Balloon's energy.
And then the moment ended, and the Balloon burst apart, each cell exploding, the entire organization of cells coming apart completely, and as the cells dissolved into dust they were hurled with such force in every direction that all of them that did not strike the sun or a planet were well launched out into the deep space between stars, going so fast that no star could hold them.
...
The Hectors marveled that Hector had to die, but now (because it was built into them from the beginning) they realized that it was good and right for him to die, that each of them was was Hector, with all his memories, all his experience, and, most important, all the delicate structure of energy and form that would stay with them as they swept through the galaxy. Hector would not die, only the center of this Hector, and so, though they understood (or thought they understood) his pain and fear they could hold off no longer. Hector, with all his memories, all his experience, and, most important, all the delicate structure of energy and form that would stay with them as they swept through the galaxy. Hector would not die, only the center of this Hector, and so, though they understood (or thought they understood) his pain and fear they could hold off no longer.
They leaped.
The leap crumbled them but hurled them outward, each leaving the rigidity of his cell structure, losing his walls; each keeping his intellect in the swirling dust that spun out into space.
"Why," each of them asked himself (at once, for they were the same being, however separate), "did they let us go? They could have stopped us, and they did not. And because they did not stop us, they died!"
They could not imagine that the Masters might not have known how to stop the leap into the night, for the Masters had first decided Hector could exist, millions of years before, and how could they not know how to use him? It was impossible to conceive of a Master not knowing all necessary information.
And so they concluded this: That the Masters had given them a gift: stories. A trapped Hector learned stories, thousands and millions and billions of stories over the aeons of his endless captivity. But such Hectors could never be free, could never reproduce, could never pass on the stories.
But in the hundred years that these Masters had spent with them, the Hectors had learned those billions of stories, truer and kinder stories than those the Makers had built into the first Hector. And because the Masters this time had willingly given up their lives, this time the Hectors made their leap with an infinite increase of knowledge and, therefore, wisdom.
As pointed out above in the comments, it looks like the story about the metal bar is "Juggernaut" (1944), by A. E. van Vogt. I'm not sure though what anthology if any would have contained both this story and the Orson Scott Card story.
