Ask HN: Any recommended online simulator to learn trading?

Don't learn trading. That's a constant drain of time requiring constant effort. Learn investing instead.

Simulations are the wrong approach if your goal is to "learn" trading. Nothing can emulate real risk.

Perhaps others could suggest low-cost brokerages--I understand Robinhood has very low trading fees.

I found the app "best brokers" kinda nice.

For android https://play.google.com/store/apps/details?id=de.elfsoft.bes...

For IOS

https://itunes.apple.com/us/app/best-brokers-stock-market-ga...

Once you have a solid understanding -> real money trading apps:

https://www.investopedia.com/articles/active-trading/111315/...

If you mean like stock market simulations with fake money, yes they're are quite a few. Some even model the real stock market in near/full real-time, just that you use fake money to see where you get. I'll leave it to you to find them.

Free simulator with a lot of historical data is the real Think or Swim platform - the on demand feature (not available in the paper trading version)