Device farm
Also called: cloud device lab, real device cloud, BrowserStack
A device farm is a service that provides remote access to many real phones, tablets and browsers for testing, without owning the hardware.
Device farms such as BrowserStack give breadth: many models and operating system versions, quickly, for compatibility and regression checks. They are the practical way to cover a wide device matrix.
They do not replace a phone in the hand. Install and upgrade friction, permission prompts, the keyboard covering a field, behaviour on a poor connection and anything involving a paired wearable are found on physical devices.
I use both and say which was used for what. A readiness note for mobile work should state the device coverage honestly: farm or physical, which operating system versions, and which journeys were walked by hand.