Forgive the ‘newb’ nature of the question, but I’ve been born and raised with full-body workouts.
With a split workout, say chest day, is the idea to try and progress in the main lift (say BB bench) and then use the secondary lifts (say incline DB, flyes etc) to just burn the muscle out, get a good pump etc, or is the aim to literally progress on all lifts?
Well I like progression, but understand you will have days when you can’t and sometimes it may be benefical to not kill yourself.
Obviously if a muscle you struggle to feel working (i.e lats) then advice would be different.
For isolation stuff I’m less bothered, i.e in the past my curls have led to do much body english when I focused on progression over a good contraction.
Like most of BB you need to be patient and try different things, evalulate it for yourself. My biceps experience/example may not be the same for you.