How to properly configure deployment for different environments and servers?

Question

git

Hello!
There are 2 branches: master and dev. Each branch has its own server (master and dev), the repository is on Gitlab. Until the commits in the dev branch are checked out, they are not merged into master. Auto-deploy is configured on the dev server and all changes that are pushed are updated there instantly. Branches are merged manually to the master server, and then git pull is done, migrations are performed for the base, also manually. With this approach, the site may not work for some time.
Tell me, is it possible to somehow automate work with the master server and how can I painlessly roll changes in this scheme? Or maybe you know a better scheme for such an interaction.