- Power Armor is attached and removed piece by piece.
- It is not really known if it is possible for the Space Marine to remove his power armor by himself, but it is typically done with assistance from a Servitor or Tech Marine.
- Power Armor can not be used as the Fallout 4 armor, it is in pieces until put together on the wearer.
Astartes power armor connects to its wearer through the black carapace interface. This is how the wearers central nervous system and mind link to the device. The connection allows a sort of power steering of all motor functions and even through the helmet interface; i.e. the Astartes thinks he wants to zoom in on that far away thing and it does it without voice command or some physical interaction.
We know for a fact that Power Armor can be removed, replaced, etc. during battlefield conditions with the assistance of a servitor or Tech Marine. One simple example of this is the 13th Company of the Space Wolves chapter. Their unit has been operating behind enemy lines for 10,000 years and scavenging enemy parts to repair their armor (typically using their blades or claws to scratch away the marks of Chaos and etc. they enemy adorn themselves with).
At this point I do not believe we know if an Astartes can remove or put on armor either by himself or another without special tools utilized by the mechanicum. Typically any time the armor is "adjusted" there are extensive rites and practices that must be observed; but such as in the case with the Space Wolves 13th Company (I'm sure they ran out of holy tech oil after the first thousand years or so), and possibly it has to do with non-codex chapters, they can be skipped or ignored as needed.
Most often a Space Marine does not take of his armor, because there is no need. All necessary bodily functions such as waste and nourishment are automatic. In addition some forces or individuals are unable to remove their Power Armor for one reason or another. Servants of Chaos are often bloated or grow grotesque features that bind their armors to their bodies or even battle damage renders an armor fused forever.