That's not just Robinhood. I'm not aware of any brokerage that lets you trade on the major U.S. markets with less than $25k.

You can trade with less. You can't day trade, defined based on the number of trades in a certain period. Some brokerages may enforce additional limits, but I've personally traded on NASDAQ via US brokerage accounts with less in them than that, so you certainly can do it.

Huh? You can open a Fidelity account with $2500, and buy any stock you want including US and Int'l markets.

Day trading has a specific definition in this context (can't remember if it's SEC, FINRA, or some other acronym), and refers to the buying and selling of the same security with a single day. That $2.5k Fidelity account will go out of its way to tell you that you're not allowed to do that.

Yes, but you can't do that four or more times in a period of five business days if it accounts for over some percentage of your portfolio.

The $25k minimum only applies to day-trading accounts. See sibling comments for details.