People spend their careers perfecting this, like me. So I'll keep it short. You can do it yourself or let the hosting provider do it for you. Letting the hosting provider do it for you will always cost more but also means the maintenance overhead is lower. Doing it yourself will require expertise which is clear you don't have so stick with other people, aka the hosting provider, do it for you as much as possible. Unity is CPU bound, memory doesn't effect much so make sure that will work for your needs. Finally they all do pretty much the same thing. Azure, AWS, and GCP are all enterprise level so they can handle anything.
With all the orchestration you can get it all setup with "Terraform", via code. Long term, definitely do this, short term might want to just do things manually until you get a grasp of things.
Spinning up servers takes from 3 to 5 minutes so its not suitable for fast paced games. That means you need to pay for running servers always then keep a reserve to account for faster scaling. GCP uses "Kubernetes" for this, Azure uses playfab, Aws uses GameLift(never used it). They all offer things i explained above.
In the end if you're using these enterprise scale dedicated servers you have a long road ahead of research. Otherwise for smaller hosting providers there are things like bluehost and the like.
This is all very hover/high level. If you want more details and it see your reply I can give more details.
I have 7+ years building enterprise infrastructure on AWS. So doing things myself is "easy" but is a lot of work.
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