This app was mentioned in 6 comments, with an average of 2.67 upvotes
I recently got the Chess King App - Endings 1600-2400 and I'm very happy with it
It covers all the different types of ending with theoretical sections and practice exercises too (table of contents here)
What I like is that it's very easy to see the lesson being taught by flicking through the moves (unlike in a book for which you need a board), plus it also shows you the variations in case you can't understand why a particular move fails. It seems quite comprehensive (certainly enough for my aspirations), and the theory section is concise and to the point. Plus it's a bargain at 8 EUR for the full version
Books are the best way to learn broad concepts. Someone already recommended Silman's, which is good.
Reinforcement and learning specific details/tricks is harder. I really like this Android app which was also recommended. Not sure if something similar exists on other platforms. It's also helpful to go over annotated GM games with endings, and also to play a lot of endings and study your own.
I've also found playing correspondence chess to be very helpful with studying endings, since endgame knowledge is such an important differentiator there.
I don't think there are any good resources online for studying endgames. I've tried ChessTempo etc. and did not think that it reinforced good habits and techniques. Playing against an engine was also not helpful, because engines will not play the best human moves in lost positions, so they won't help you learn how to not to blunder won positions into draws.
The best endgame practice resources I've found have been CT ART's endings app, Chessimo on the deskop, and using a database to search for endings of a particular type and then playing guess the move with the master that won or held a tricky position.
That's for practicing. For learning endings, the best resource is still books.
EDIT: Also, of course, playing lots of games and analyzing afterwards is great for training endings as well (and all other parts of the game).
https://play.google.com/store/apps/details?id=com.chessking.android.learn.totalchessending
You have to purchase the full content in the app but it is definitely worth it. Huge amount of theory + exercises. And you can be offline to use it.
There are many different kinds of endgames. King and pawn endings are the most basic. I guess rook endings is the next step above that.
I learned practical endings from this book which looks like it's out of print now, unfortunately. Silman has a book that's also supposed to be very good.
There are also specific books that dig into very particular aspects of endgame play, like this book on pawn endings that heavily focuses on "corresponding squares". That book helped me a lot.
For practice, I found this android app to be very helpful. I believe there is also a PC version and probably an iOS version as well.
Playing endings out against a computer can be helpful but there are big downsides, too. The computer will usually not find the most stubborn human defense, for example. If a position is objectively lost it will just play any trash move because it sees everything as equally losing.
In addition to all of this, there's always the psychological aspect of chess -- meaning, it helps to keep in mind what you're actually trying to accomplish and to learn mental tricks to make playing endings easier. For example, sometimes it helps to imagine rearranging the pieces or pawns into a winning position and then figuring out how to work backwards from that. Also, it helps to keep in mind that zugzwang is a common factor in endings. When you're trying to checkmate somebody, it helps to visualize a mating "net" around the king, like imagining the squares he can't move to as being highlighted. There are also specific tactics and patterns that come up over and over again in endings.
It's a big subject!
I find 'Chess King' mobile apps are excellent. I particularly recommend this one..
https://play.google.com/store/apps/details?id=com.chessking.android.learn.totalchessending