Git Deployment Multiple Existing Environments

Hello, I am kind of new to Git and I have a question regarding using Git for a website. I have searched a lot but haven't found a solution yet. We already have 3-4 environments setup on our Windows servers without Git and each environment already has code which is different from each other.

There are three environments Live UAT Test (has the latest code)

And then developers have their local copies.

We write and test the code locally and manually move each point from one environment to other using merging software and test at each environment. Now we want to use git because manually moving the code is a lengthy process. Also as the developers have local copies, it is very difficult to manage code.

Code is written locally by the team and then after testing locally it is first merged with "Test" environment code, then "UAT" and then, finally with "Live". So we have two concerns:

There is different code already existing on these environments. Testing the code on each environment using the web server.

What is the best way to go about it? As I am new to git more details will be helpful, like commands to use.

Thanks,