> btw, I don’t know how I actually got these invites. Either from math competitions, or through my math teachers who found a way to get rid of me ;)

Almost certainly from the math competitions. Nikolay Konstantinov [1] had his apartment full of boxes of punched cards with data on every high school student who produced at least a partial solution to any of the advanced problems at those competitions.

[1] https://en.wikipedia.org/wiki/Nikolay_Konstantinov