Why are there separate Bitcoin and Bitcoin Core GitHub repositories?

Can someone explain why these two repositories exist at the same time?

https://github.com/bitcoin https://github.com/bitcoin-core

For someone new to the codebase, it can be a bit confusing as to what to actually trust. Also, is there a good introduction explaining how access to this repository is managed? Who is entitled to approve a merge request, and for what reasons can this right be revoked?

thank you!