You'd push one of those buttons in to select a station and it'd stay pressed. When you push another button in to choose another station, the previously pressed button would "pop" back up/depress (thereby "unselecting" itself), thereby enforcing the mutual exclusivity of a station selection.
Preset radios came in a huge variety and some of them were actually round. Car radios (the radios that have stuck around the longest) need big and bulky buttons so you can operate them without taking your eyes off the road, but some home radios had smaller, circular buttons instead.
The "radio" buttons I used as a kid were small, rectangular sticks poking out of a metal plate that you pushed in. Every brand had their own shapes and designs. Unfortunately, I can't find any pictures of older radios on the internet because every image searching site seems to have been overtaken with badly generated AI art, stock photos, and cheap, plastic "retro vintage" Amazon listings.
0: https://en.wikipedia.org/wiki/Radio_button