The honest answer is that it depends, and the things it depends on are knowable. It depends on the muscle, on how hard you hit it, and on how long you've been training. Here's how to think about it, and how the app just works it out for you.
For most people, a muscle group trained hard needs somewhere between one and three days before it's ready for another hard session. Smaller muscles that do a lot of work anyway come back at the quick end. Big movers that took a heavy compound session sit at the slow end. Your lower back is usually the slowest thing in the building.
Recovery is not one number for your whole body. Core and forearms turn around fast because they're built for repeated low-load work. Quads and glutes after a heavy squat session need considerably longer. This is why full-body-every-other-day and a five-day split can both be correct, just for different muscles.
A back squat and a walking lunge both hit your quads, but not to the same degree. A muscle that was the primary mover in a heavy compound needs meaningfully more time than one that was stabilising. Programming that treats both as "legs trained" is throwing away the distinction that matters.
Newer lifters need more rest, not less. Work capacity, technique efficiency and the sheer novelty of the stimulus all mean a beginner's session costs more to recover from than the same session does for someone five years in. Recovery gets faster as you get more trained, though the gains taper off.
The adaptation you're chasing happens during the rest, not during the set. A rest day is when the work you already did turns into something. Skipping it doesn't get you there faster, it just means the next session starts from a worse place than the last one did.
Workout With Me starts a recovery clock on every muscle you train, sized to your experience level and to how hard that specific muscle got hit. It brings a muscle back into rotation once it's 80% recovered, and if nothing is ready it says so and tells you when to come back. You stop doing the arithmetic.
The time a muscle group needs after training before it can productively be trained hard again. It varies by muscle, by how directly the exercise loaded that muscle, and by the lifter's training experience, which is why a fixed rest interval for the whole body suits nobody exactly.