Originally Posted by mack_turtle

my mountain bike uses 29" tires which fit on a 622mm rim, aka 29er, aka 700C.
my (gravel) road bike uses 650B aka 27.5" tires.
both are tubeless but I could easily use a 26" tube in an emergency if needed. tubes stretch. within reason, you can use a somewhat undersized tube in a larger tire. I don't think it works well the other way, trying to cram a fatter tube into a narrow tire. I have not tried it though.

there's some hyperbole from riders and marketers about which is "better" but the truth is that the best choice is subjective and depends on the riders, the terrain, and the bike.