My preference is to learn from 1) other musicians, 2) recordings, 3) sheet music, in that order.
I have problems remembering lyrics unless I write them down. I'm generally a visual thinking type and can often "see" the page of lyrics in my mind when I'm singing.
Repetition is really important for me. Some songs stick in my head easily but most of the time I need to hear, play and sing songs a lot before I know them. I too, use the car to learn songs. The shower's OK but personally, I'd have to use it solo (sorry Alice) for learning songs, otherwise I'd probably get too distracted. BTW, what time signature should you use for shower duets? Is it OK to change timing in mid-stream? Should you put your instrument in a case? So many questions...

Dave T