Basically, I just want to know if there’s any easy way to use websites to practice endgames and openings by just opening my browser. I keep finding downloads to Windows programs, which is a problem when I play chess on mobile devices. Then there are the options that require to pay, which isn’t really something I could do.
Three options I can think of immediately:
https://chess-endgame-trainer.firebaseapp.com/home is a fantastic chess endgame trainer.
Chessable also lets you create your own courses (for free) which you can then do spaced repetition on.
Listudy is another option.
I would strongly suggest looking into chess.com as they have an extensive "Learning" module on end game and end game tactics. You are limited to the amount of modules you can complete per day unless you are willing to pay for a subscription. This isn't necessarily a bad thing as learning to much can result in concepts not necessarily taking hold.
Alternatively lichess.com has study section which is comparable but in my opinion not to the same standard. That being said it is free so might be a solid option. Those would be the two I would strongly suggest for web based.
There is another option which is to look into programs like ChessBase or SCID and look at creating positions of end games and trying to play them out with computer analysis. That is a fairly advanced method of doing this though and I would suggest that you do that once you feel well established.
One of the best EndGame trainers I have seen was in ChessMaster GrandMaster Edition with a learning course by Josh Waitzkin which is narrated and well presented.