If I’m doing total body workouts like you said upper one day lower the next I usually split them up with a day of cardio/ rest in between and alternate like so
Day1 - upper body. Day8 - lower body
Day2 - cardio Day9 - cardio
Day3 - lower body. Day10 - upper body
Day4 - cardio. Day11 - cardio
Day5 - upper body Day12 - lower body
Day6 - cardio Day13 - cardio
Day7 - rest / clean up anything I feel I didn’t hit good like abs forearms traps
Then the next week I alternate upper and lower body so that I do lower body twice and give upper time to rest Cardio can be anything from HIT to treadmill on every exercise even CARDIO!! You have to move up either weight or reps for weights
Speed or incline for cardio always better yourself
If I’m focused on body parts I do it backwards to keep from having to fight for machines on national chest day

and I like to fry all my helper muscles before chest that way I can’t cheat by using other body parts
Day1 - legs + cardio
Day2 - back + cardio
Day3 - arms + cardio
Day4 - shoulders + cardio
Day5 - chest + cardio
Day6 - clean up and cardio
Day7 - rest rest rest