What is coming from a backing track and what has been recorded by yourself and/or members?
That's a good question! I'm going to give a long, detailed answer because you are interested in the process and have the background to do this and more. I record a lot of songs. They are either originals or sax learning exercises like Sea Cruise. Many, like this one start with a chord chart entered into Band in a Box. BiaB can produce multiple audio tracks, so I now use that every time. For jazz, the rhythm sections are often good, with real tracks. If you have a good library, MIDI sounds can also be very good. From the three to five tracks BiaB produces, I import them to Logic on the Mac, but Windows and Linux also have many alternatives. After messing with the levels, I decide what I want to keep. I often end up throwing away one of the two guitars, as I did here in Sea Cruise. The rest is specific to Sea Cruise and your first question.
I did a version in Bb which sounded good to me on alto for certain notes, but when I went to add a vocal, I found it was too high for that. When I went back to BiaB to change the key to F, I also added an intro, which I felt was needed. Back to Logic, I then recorded some guitar for the 4-bar intro and a few embellishments in the first verse. Then I added the vocals, one verse at a time. Cover songs are for fun, and not being much of a singer, I spent very little time on this part.
When Nick's track was available I imported it into the Logic file, syncing the count on his file with the one on the BiaB drums. I picked some parts of the song I wanted to play on, and muted Nick's bari to do that. I played along with his bari at the very end. At this point I did a few volume adjustments on the piano. This is where my own lack of mixing talet probably shows through, because the moves are like a clumsy dancer, but I get it still makes for a little variation in the total sound. Finally, I did something I've never done before. I though this song could use a "top 40" treatment, where a limiter is pushed up too high so the constant energy of the song stays high.
Sometimes, I fly in better drum tracks. This is only possible because BiaB produces a track that never varies in tempo, like a real band might.
Other times I'm forced to fix the awful BiaB uncontrollable endings. That often requires adding a few bass notes, piano chords, guitar riffs.
I believe that may more than you wanted, but I figured if we're going to travel this road, I may as well show you the sights