How do phone farms work?

Let's say I have money to spend on a phone farm or even renting it through someone, and I would like to use it to have an army of accounts to use for following.
How would I go about it?

I build a phone farm that i can access anytime, anywhere with cloud phones，which
provides AI and automation tools, like the synchronizer and RPA, that can help me automate repetitive tasks.

like this?

I'd first determine how big of an army you want to create. Based off of that decide whether it's worth setting up in-house/ renting out someone else's hardware/just paying for the service.

Since building it from scratch in-house is quite the hassle. You need to buy/set up hardware, have software that allows you to do what you want to do, and set up a reliable network infrastructure, a bunch of stuff really.

What's the specific use case / platform?