Maybe a dumb question about strength training

Anything that challenges your muscles to do more work than is normally demanded of them can be considered strength training.

Exercise in general is a stress upon the body. When you're resting between workouts, the body repairs itself to recover from these stresses and thus you become stronger.

Some of the best exercises use only your body weight as that forces many of your smaller muscles and your core to activate and work as stabilizers. If you're staying true to your form and pushing yourself through the workouts with enough resistance that you're fatiguing your muscles towards the end of your movements...it's definitely strength training.

Cheers!

I would classify those as circuit training. If it was true strength training there would be more breaks and rest periods, and you would not do the same muscle groups within 48 hours of working that same muscle group. Muscles build during the rest period 48+ hours, after getting micro-tears from the strength training, without the rest your muscles would actually deteriorate, and you would lose strength.

I also would not consider something strength training if I could do more than 15 reps without a rest (push-ups and squats could still be considered strength training, but if you can do a lot of reps with body weight only, it is better if you add weight and do less reps)

This is indeed strength training. The only difference between it and traditional strength training with free weights or on a machine is the intensity and duration (i.e. few/no rests, lower weight/higher repetition) and that it uses your body weight. It's a perfectly good way to increase your strength, some say even better than just lifting weights alone because it trains your full body in concert, rather than just single muscle groups.

Explain to me how a squat or a deadlift trains a single muscle group? Unless you are isolating the muscles with a machine of some sort there are a lot of extra muscles working to stabilize the body. Each workout might target a specific large group, but it does workout many smaller groups at the same time.

I would call that a circuit HIIT style, not strength.

Strength training requires two major things.

1. Resistance at about 80% or greater of your maximum effort.
2. Progressive overload. i.e. systematic and consistent increases in resistance.

You may gain some strength through other means and methods, especially if you are a beginner but those gains will not continue unless the resistance is high enough and you are progressively overloading your muscle.