This looks somewhat like Still River by Hal Clement.
the protagonist is a girl or a young woman.
Check. She's called Molly (Mary Warrender Chmenici) and is one of five Galactic students being sent to Enigma-88, a strange planet used as a test resort for students.
She has a friend who is an alien and he sleeps on a ladder, not a bed. The Alien, has a lot of tentacles and is an octopus type being.
Almost check. She has one friend, Jenny, a Rimmore from Hrimm, that is centipede-shaped and sleeps on a ladder. And Charlie, a Kantrick, who has prehensile limbs sprouting from a sack-like body protected by an exoskeleton. Molly's best friend is Joe, a Nethneen, with a spherical body and four sort-of-tentacles, two adapted for gripping, two for locomotion. The Nethneen prehensile organs are described as "delicate tendrils".
She has a bag (or something like a bag) that is bigger on the inside. She and her friend use it as a house sometimes, I think.
Almost check. They all have foldable pressurized housings they use throughout their exploration.
At some point they end up on a hollow planet where the atmosphere and life are on the inside.
Check. The planet Enigma-88, which, they discover, is hollow.
Also gravity is reversed (it pushes outwards rather than pulling down). They could look up and see stuff hanging on the ceiling.
Does not check. There is a scene with stuff on the ceiling, but gravity inside Enigma-88 is not very remarkable. What gravity there is of course pulls downwards.
There also may have been portals that they traveled through, but I'm not sure.
Almost check. There are "Leinster sites", that are where the interstellar travel can happen through "false space". Enigma 88 is a short distance out of the Eta Carinae Leinster site.