In terms of ear training recognising all 12 intervals by ear and in their compounds to 3 octaves is only the start.

You might know them all but in music it's about recognising the many movements between them.

From Perfect 5th, maj6th then maj7th a chromatic passing note between the 5 and 6, or was it between 6 and 7? This caught me out as the underlying diatonic movement is the same.

Playing by ear is a whole other thing and by taking it on I realise how much work there is to be done like I am a 4 year old who has just learnt the alphabet.