I don't understand this quote from Capablanca

I've read (don't remember where unfortunatly but found the quote on google) that "In order to improve your game, you must study the endgame before anything else; for whereas the endings can be studied and mastered by themselves, the middlegame and the opening must be studied in relation to the endgame"

Espescially the second part. What does this mean? That one should know what endings he feels comfortable with in order to orientate his middlegame planning and his opening choices? Or that a player should master the endgames to know at every moment what he should strive for in the middlegame...?

https://www.chess.com/forum/view/general/why-you-should-learn-endgames-before-openings

Entertaining thread in the link, but practically speaking ...

Would you rather be the one with a quick concrete material advantage early in the game or the one who is objectively lost except for technique which will take several dozen more moves and your opponent’s cooperation (hope chess?)?

Would you rather lose your decisive winning advantage on move 15 and be able to analyze exactly where you went wrong and correct it, or would you rather win on move 45 when the engines and masters say you should have lost on move 10, then try to analyze what you and your opponent would have played in your fantasy-land had you made one of the 28 better moves available on move 10.

Would rather have a 2-0 lead in a best-of-seven series or be gifted by the gods a touch of the slight clutch at the cost of having to spot your opponent a 3-1 advantage?

I don’t think Capa was talking about players who can’t make it to move 10 (15, 20, etc.) without being at a decisive material disadvantage. Until then ...

That's pretty simple: to evaluate most opening or middlegame positions, you have to be able to evaluate any endgame that may arise from them.

"... The game might be divided into three parts, i.e.:- 1. The opening. 2. The middle-game. 3. The end-game. There is one thing you must strive for, to be equally efficient in the three parts. Whether you are a strong or a weak player, you should try to be of equal strength in the three parts. ..." - Capablanca

It is recommended to study the endgame FIRST because you'll need that knowledge to convert KR vs K or something similar.

I understand the link of endgame with the middlegames. But i don't with the opening. Non GM can assess the endgame depending on the opening they choose? I don't know what kind of ending i'll end up with when i play the English or i defend with an indian defense... I know most of the middle game plan (in some ways)

All of this worked for Capablanca 100 years ago, playing classical openings at classical time controls. It certainly has less relevance to rapid/blitz games played in an era where computers are coming up with viable continuations that run counter to the traditional principles.  But, even today, many opening variations are best understood through the likely endgame positions: Advance French and the Exchange Grunfeld to name a couple of examples.  Also, openings that often lead to an isolated q pawn.  Of course, if Capa was alive today, computers would have "solved" most of his traditional endings, and, although he'd rarely lose, he wouldn't win very many vs his top competitors.

I play a positional style of game.
If I get a pawn up, I start to simplify and must know an endgame strategy.
1 pawn = Queen.

It seems to me that the real question is about how much endgame study to do before looking at other subjects. I don’t know that there is much general agreement about that, but I think that Silman indicated one plausible answer:

"... if you have just learned to play, all you need to study is the section [of Silman's Complete Endgame Course] designed for beginners (Part One). After mastering the material there, put the book away and spend your time studying tactics and a few strategic concepts. Once ... you feel you're ready to break into class 'E', re-open this book and master the material that's been designed for players rated from 1000 to 1199 (Part Two), then repeat this process as you move up the rating ladder. ..." - IM Jeremy Silmam in his 2007 Complete Endgame Course

if u know what kind of endgame u prefer, than u can match it with midgame then opening. as example, if u like to mate king with many pieces, u should start recollect ur pieces at center. and in the opening phase the kind of opening that suit to recollect pieces such as Knight game or French defense

from capablanca view, that's was the way to Learn chess by relating endgame>midgame>opening. this happen before chess engine era when modern/new generation chess player depend too much rather than pursuing own style and thinking

I don't disagree with the importance of endgame, the debate isn't there. Like i said, it is to grasp the direct link between opening and endgame (not middlegame and endgame since, like said, it's easier to understand the link). So i find in interesting to read that some mating pattern are more common with certain openings like the french

He wasn't very fond of that.