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All the CGs unlock automatically once you finish the Side B ending.

But there's not that many missable scenes before that - it's really only if you don't run into either the third Meleuma or Greenhouse encounter in the industrial area (which are also needed to start the last optional chimera scene); which if you haven't done triggers the message about missing something.

Other than that, everything is on the main path.

If you meant a guide on how endings work, that was covered in the release devlog - hopefully that helps.