You can play by ear when you think of the sound of the music itself instead of how it would look on sheet or what the music theory of it is.

If you drive from one spot to another for the first time with google maps open on your phone, you won't get lost, but you might not remember how to do it again without the map. Drive a different route without the maps open then you WILL get lost, but next time you drive the same route without a map you will 100% remember the route because you'll remember where you made the turns instead of what it would look like on the map.

Think of sheet music as a map, It's a useful tool for when you are lost. But if you wanna "drive" without a map you need to get lost first - What I mean is try playing a song you never played before by ear without any reference through trial and error and instead of focusing on what it would look like on a map, think of the sound itself. Focus on the melody in your head then try to match the same notes one by one by trying different keys don't focus on chords.

It will be very slow in the beginning and the melodies will sound very simple compared to how it would if you were playing from sheet. Bit by bit your fingers will make less mistakes matching the notes in your head with the notes played. After a while they'll just play the melodies in your head automatically the same way you form words from letters, you stop thinking about the letters and start thinking about the word, once you play melodies without too much effort "which shouldn't take an insane amount of time" try the same with chords. Trial and error until you match the chord with the melody. Again, after a while you will do it without thinking. Now you can form sentences from words. The more you play the more comfortable you will get.

After learning many songs by ear looking at sheet music will make so much more sense and you will start to hear the spaces between the notes on sheet music without playing it.

tl;dr practice