I usually forget intros and first lines but once going it comes back. I've played by ear for as long as I can remember. I'm also classically trained so I can read as well although I'm not that good.

I think I learn tunes in terms of intervals and chord "shapes" because it I know a tune sufficiently to have a go at playing it at all then I can play it in any required key. Strangely can't do this with classical stuff that I've memorized.

You can train your ears. Until my early 20's I had real trouble decoding jazzier chords especially where the bass bears little relation to the rest of the notes like 9ths, 11th and such. This changed after I got an Elton John songbook. As he uses a selection of such chords I found that playing songs from the music enabled me to "hear" the chords. Admittedly we are not talking Oscar Petersen.....

What this also taught me was that a lot of pop sheet music, and indeed chords you find on the web, are wrong. Not necessarily by a lot,but wrong nevertgheless. This includes that Elton John book, by the way. That must have been someone's transcription of the songs as Sir EJ doesn't read music.