I'm not sure even this question is off-topic or not. But bear with me. Currently I have a good knowledge of bitcoin protocol and network but know very little about its source code and implementation but want to contribute to the project. So I started reading bitcoin-core source code and learning about it. One of the first steps is reviewing other pull requests. I want to know is it okay to ask my very basic questions about source code as comments in reviewing these pull requests? Or these basic questions may be better to asked in another place? (such as IRC chat rooms)
I think you'll get different perspectives on this from different people depending on how "basic" we are talking, the PR author and whether the PR is high priority and already obtaining a lot of review from others. I'd recommend asking questions about any PR on Libera IRC (#bitcoin-core-pr-reviews) and attending the weekly Bitcoin Core PR review club sessions first. This will help you gauge which questions should be submitted as comments on a PR to help move the review of that PR forward and which questions should be asked on IRC for your own personal understanding. Certainly general questions on source code not being changed by any particular PR should be asked about on IRC rather than on a random PR.
Generally if you submit a review comment on the PR explaining what you've looked at or tested the PR author will appreciate the effort and answer a question or two. Obviously taken to extremes lots of basic questions on the PR aren't going to help move the review of that PR forward and should be taken to IRC or here on Bitcoin StackExchange.
For additional guidance on reviewing PRs see this doc from Jon Atack.