With ChatGPT, you can create a project and, using the online UI, upload files to that project. This can be useful when you want to have a common set of documentation to use with the individual chats under that project. For the individual chats, you can also attach files for it to refer to on a more temporary basis.
With Claude Code, you have have it reference documentation within your project. In that case, just create a folder to contain the set of docs you want it to use.
I have used both of these approaches. For both, give specific instructions to the AI to refer to those sets of documentation for whatever task you ask it to perform.
