how do I study endgames with limited time
Best way (IMO) is to study them 1 at a time- the one you just played. When you play an endgame in a game. Study similar ones is a manual. Then set it up vs a computer and play both sides until you understand that one.
I think it depends on the desired outcome. To improve otb or play exclusively online? You state limited time, not financial resources.
If the former, sorry, books are the way to go and with a physical board set up. I'd recommend Chess endings, essential knowledge, Yuri Averbakh paired with John Nunns Endgame workbook for kids. This is how I allocate 20/30 mins when 'spare'. Silman, for example, has too much "waffle".
If only online and you're determined not to buy books - I don't know . Probably as SwimmerBill recommended along with 'some' program/engine to test against and drill. Exercises where you have to think (rather than online 'click and hope' style) aid memory better.
Maybe this.
https://shop.chessok.com/index.php?main_page=product_info&products_id=218
Chess Endgame Training program.

