How to develop rotating proxies for my app?

Hello all experts
For a long time i try to find out how do all those proxy providers, can someone share where i can learn to build it ?
Where do they buy the IP's ? what hosting they are using ?
( im a developer so no problem here ... )
Thanks

You could always become a reseller from existing proxy providers. I've seen several that offer an API that you could build out your own service.