So, If I understand you correctly, you have sent an email to approve/reject (I presume you are using a powerautomate approval process for this), and you want certain actions to perform after approval, these actions are in power app (under a button click) rather than in power automate it self. In that case, you have to bring those actions/functionality back to your power automate. No other option.
If it is some thing like you have to replicate the same functionality (approval process) in a canvas app as well, then also either you have to replicate the logic in both powerautomate & power app or remove the logic totally from the power app, build a separate child flow and use that in both the places.
If I understand you wrongly, sorry - please provide more details, may be pictures (if it is possible) would help to understand more.
Thank you
-Srikanth