Web Development Using Git: How to test git branch? How to merge mysql tables?

I am running a git repo for a website. I want to have a production/live website and specific number of development websites for each developer like this:

• production/live: www.mywebpage.com --> master branch --> database0
• development01: www.dev01.mywebpage.com --> dev01 branch --> database1
• development02: www.dev02.mywebpage.com --> dev02 branch --> database2
• ...

I already configured apache, everything is working fine. The databases1 and 2 are copies of the production database. Every developer will now develop a new tool or feature for the production system which requires database manipulations.

My questions are: - How does a professional web development in this case look like? - Since every developer has to add or change new database tables: How do they merge mysql tables? - How can I view the changes of the developers? How is the domain connected to each branch and database? - What recommendations could you give me?

Thank you very much for your support.