In world c, there is a new item I understand to be called a platform. Do I need to do something specific to be able to pick one up?.

The platform is in certain puzzles where the platform icon is present. It looks like this: https://talosprinciple.fandom.com/wiki/Platform
It is used in combination of recording. Record yourself taking the platform and going to places, then replay and jump on the platform, and let your former self bring you somewhere.

I can see them but I can't interact with them. Is there something that needs to be done first in order to access them?

Yes there is. Found it at the end of this building near doorway 6. Another fill in the rectangle using pieces puzzle.