I add conditioning days to the lighter or more moderate days, mostly from a length-of-time standpoint, both in the gym and in the time it takes to recover. Conditioning workouts, if done with high intensity, are super tough! Overloading my system on heavy days + conditioning is too much; I’d never recover properly to produce enough force for my lifts (with sound movement patterns, anyway) the next day, plus I’d just feel terrible, which is rarely (heh.) the goal.
Placing the conditioning workouts where we will be able to both drive the intensity highest (lighter lift days or days where the posterior chain is activated and primed for movement) and recover in time for heavy lifts is vital to being able to achieve both strength + body change goals. I find many programs run into trouble by trying to do too much all at once, leaving you exhausted after a week or two, and giving up after three, leaving you pretty much right back where you started (we’ve all been there, myself included.).
Finding a balance is huge. This is what works for me and many clients, but all programs are adaptable to your individual needs. Working in the realm of high intensity + building strength is where the magic happens. Figuring out strategies to do both in the context of real life was the game changer for my consistency in workouts, because I was actually having fun AND seeing results.