In order to understand how to get stronger, I think it's helpful to to start by defining strength.

Strength is the ability to produce force against an external resistance. Simply, strength is force production. The person that can produce the most force is the most strong.

Someone who has enough force production to squat 150 pounds is stronger than someone who can only squat 100 pounds.

Someone who has enough force production to bench press 200 pounds is stronger than someone who can only bench press 135 pounds. It's black and white.

If you want to get stronger, you can't just be exercising at a gym. You have to be constantly increasing force production.

It doesn't matter how many lunges you are doing or leg extensions. Yes, you may get hot and sweaty and feel sore the next day but if you are not increasing force production then you are not getting stronger. You are just exercising.

If you want to get stronger, force production must be increasing.