On an Android phone
Open this site's checker in Chrome. Most Android phones answer with an Adreno name, which is Qualcomm's graphics, or a Mali name, which is Arm's. Samsung and Google phones may show other names, depending on the chip inside.
To see the main chip, go to Settings, then About phone, and look for the processor. The model number in the same place lets you look up the full specification on the maker's site.
On an iPhone or iPad
Safari says Apple GPU on every model and does not name the chip. Open Settings, General, then About to see the model, and look the model up on Apple's technical specifications page. It lists the chip, such as A16 or A17, and the graphics are inside it.
Why it matters
Mostly for games and apps that list a minimum chip. Knowing the chip lets you compare your phone with those lists.
Can it be upgraded?
No. The graphics are part of the chip, which is soldered to the board.