If you just google ‘Drive for GrubHub’ or ‘Drive for UberEats,’ you should find the application page. Depending on your market they might not currently be hiring. In which case they usually have a waitlist.
You don’t need separate phones. They’re separate apps that can all be accessed with your phone. You need a phone with some power though. I use an iPhone 8. Android works too but make sure the manufacturing date is 2018 or higher. I run 8 apps while I’m working: DoorDash, GrubHub, Postmates, InstaCart, Stride, Apple Music, Maps, & Medium (while waiting in the drive-thru at TacoBell).