I'm new to this forum, so "hello" to all of you. My question about transcribing is this: Are we talking about transcribing the notes in a song or the chords. From what I've read, it sounds like both. Is that correct? Thank you.
Hello

and welcome.
Yes, I think it is both chords and notes. If I remember correctly Justin recommends starting with chords. That is how I started and it was easier for me than transcribing the notes. I still find it easier to get the chords to a simple song than it is to get the notes. Of course, I have to know the chords first and have played them a lot. If they are unfamiliar chords I will maybe just get the basic chord, but not the extension or the inversion. The voicing, I find, is easier to get. Don't know why.
The notes are a lot harder for me, and take more work. But I guess if I did not know my chords then the notes would be easiest to get.
So I would say start on whatever is easiest for you - notes or chords - or try both.
What I use a lot for practice is simple tunes I hear on You Tube, like old swing songs. I recently worked out "memories are made of this" as sung by Dean Martin. That sort of thing is a lot easier than some of the songs I try.
Even if you know the chords I think it is useful to try to play the melody or the vocal line on your guitar. Again, I recently worked out the vocal line for "with a little help from my friends" by The Beatles. That is also a simple one, written for Ringo to sing.
Working out so many simple tunes is really encouraging and good ear training.
Learning your scales is very helpful too. In fact if you play your scales a lot you will start to hear familiar tunes in them as you play around with them. One thing I found for myself is that I was worrying far too much about finding out what key a song is in before I started to transcribe it. Now all I do is I pick out the melody or vocal line without thinking about what key I might be playing in. When I get the notes, then it becomes obvious what key it is in.