New lifters get stronger at a movement by doing similar big exercises, even if they don’t do the exact movement, because their brain learns to move better.

In untrained individuals, compound movements can improve performance on isolation movements due to enhanced neuromuscular coordination, overriding the specificity principle; however, in trained individuals, strength gains are highly movement-specific.

Whether you train with one joint (like leg extensions) or multiple joints (like squats), you get just as strong at the movement you train — but if you train a simple movement, you get a bit stronger at holding still too, compared to complex movements.

Lifting lighter weights many times until you can't do more builds muscle strength just as well as lifting heavy weights a few times until you can't do more, for people who already work out.

Performing resistance exercises with either isolated or compound movements can increase maximum strength not only in the exercises done but also in other unrelated movements, suggesting that the nervous system adapts in a widespread way.

Lifting heavy weights for just a few reps makes you stronger in exercises that work one muscle at a time, more than lifting lighter weights for many reps, especially if you already work out regularly.