Assuming that their asking how to get Linux on an "off-the-shelf phone" means they want to flash Linux on a device that doesn't come with it, it's very possible to flash a custom ROM on lots of Android devices. Projects like LineageOS have a very healthy, active userbase that has ported their ROM to many different models of phone. Lots of Samsung devices can be used, some Nokia devices, nearly every Google Pixel device. There are plenty more, those are just the ones I remember.

The caveat is that your phone has to have an unlocked bootloader; if your phone is provided to you by your carrier, it's almost definitely not unlocked. Unless you've finished paying off a carrier-provided device, you can't unlock it yet.

I wanted to flash LineageOS on my G Pixel 6, but the bootloader is locked down.

EDIT to add: custom ROM does not just mean custom version of Android. There's postmarketOS, Ubuntu Touch, Plasma Mobile, Mobian, and more that are Linux, w/o any hint of Android. SailfishOS got a lot of attention awhile back, but it's not open source, and it's not free.

LineageOS and a couple of the others have the ability to run Android app, sometimes including the ability to add the play store and other Google services to their ROMs. Of course, those are all built on top of Android (which is still modified Linux) so it depends on how far "just Linux" you want to go.