Consider a typical bodybuilding leg workout that consists of multiple machine exercises combined with some barbell exercises, say for example the following
Seated leg extensions
(Smith machine squats)
Barbell back squats
Front squat machine (facing front/back)
Pendulum squat machine
Lying leg press/Seated leg press
Seated leg curls
Lying leg curls
Barbell Deadlift (variant: Regular, Sumo,...)
The order of exercises may vary a bit, but the principle is often the same that you have a combination of machine and free weight movements.
If one asks, of how to substitute for example leg extensions with free weights, one often hears that you should do just squats. However one part of the program above is already squats, so it doesn't seem to make sense in this case.
So, what would be good options to replace the non free weight exercises above with free weight ones (this may include barbell exercises, dumbbell exercises, bodyweight with minimal equipment or also landmine exercises including a landmine attachment like pentagon bar for heavier loading). The variants should be safe and you should have a good control about the load.
My first try for for the replacement list would be:
Smith machine squats: Just regular squats
Pendulum Squat machine: Same replacement as Front Squat machine facing backward
Lying leg press: ??
Seated leg press: ??
Lying leg curls: First I thought of nordic hamstring curls, GHD-curls, swiss ball curls or something like that, but I don't see how to get a finer control over the load in this case. Also for example in swiss ball it is not so easy to maintain a bit of hip flexion as explained here, #active insufficiency
Seated leg curls: ??
Would be great, if you could explain in your answer your exercises selection a bit with some finer details (for example why to replace the lying leg curl with exercise x but the seated one with exercise y).