Strength is the muscles capability to produce force. "Raw strength", if such a term can be used, highly correlates to the cross-sectional area of the muscle. But improving the maximal amount of weight that can be lifted in a particular lift is more than strength, it requires a high degree of skill.
Skill shouldn't be confused with technique. Skill does involve technique but also neurological factors. Consider the friendly competition between bodybuilder Tom Platz and top level powerlifter Dr. Fred Hatfied. When measuring who could do the most amount of reps at a given weight, Platz smoked the world record squatter. However, Platz was nowhere near capable of squatting the 1000+ lbs "Dr Squat" did. It's a valuable lesson. In order to lift as much as possible, the skill of lifting limit weights must be practiced.
The fastest increases in a persons 1 repetition maximum (1RM) occurs with weights above roughly 85% of 1RM. It's at this point skill can be practiced optimally. Lighter weights don't require enough inter- and intramuscular coordination and it's easy to "cheat" with technique by avoiding correct muscle activation.
There will be an upper limit of weight at which technique breaks down and the lifter is at that point training the wrong movement pattern. This upper limit is individual but in general, the more experienced the lifter, the higher the upper limit should be. For instance, a beginner can have technique break down significantly already at 80% of 1RM, which shouldn't happen with an experienced lifter as long as they’ve trained correctly. For experienced lifters it's not uncommon to see technique breakdown only happen at over 95% of 1RM. It is however common to see it at 100%. It might not look severe, perhaps a little greater forward lean in the squat or slightly wrong bar trajectory in the bench press, but those small things make a big difference at a higher level.
Thus the general recommendation for the intermediate lifter is that in order to improve skill (and thus competition results) the most effective weights tend to be between 85-95%. For beginners it's more a matter of "play it by ear" and lowering the weights when the weights become too heavy to manage perfectly.
"The closer the lift biomechanically, weight used, repetition range and rest between sets is to the competition lift - the more specific the skill practice is."
- Everlifting Modular System, Core 1.1.
Thus the most specific skill practice is emulating competition with single repetitions at near-maximal or maximal weights in the competition lift. Don't make the mistake of assuming this should be done all the time, however. Doing the same thing over and over leads to habituation, in other words, adaptation stops. In reality a variety of rep ranges are used and often a variety of lifts as well. More on that later.