> No one will sell you any stocks for $1,

You have been able to buy mutual funds in dollar increments for like forever, with the significant caveat that there's often a minimum purchase amount, but once you hold that you can add in single dollars if you want +/- they only deal in a certain number of decimals.

Fractional share trading of stocks and ETFs happens at a lot of brokers for the past several years, too. Do they really want you to buy $1 of BRK.A? probably not, but you probably can.

Also, there's like penny stocks. :P

But yeah, there's no mechanism for that $1 to turn into millions of dollars within a few days. It can happen with a very well timed purchase, many decades, and some inflation, but that's not very exciting.