One of the key things that often prevents people from developing functional strength vs mirror muscles / beach muscles is neglecting the posterior chain.
Basically these are all the muscles on the back side of you, the ones you don’t see in a mirror. Part of it is likely just a out of sight, out of mind issue.
But the other part is those muscles are a little harder to train, especially without at least some equipment. As a result a lot of home workout programs manage to do a decent job working the anterior chain (muscles on the front of you) but neglect the posterior chain.
The reason they are harder to train is in general these muscles work the “pulling” side of things, where as the other side of them is the push. Using your weight and the floor gravity can help give you resistance for pushing exercises, but it is a little harder for pulling.
So what’s the problem?
The problem is strength training is all about balance. Each muscle has an opposing muscle, your quads to your hamstrings, your biceps to your triceps, etc. One pushes, the other pulls.
When your workout stresses one side and neglects the other you risk injuries doing ordinary things. In the “real world” most things involve rotation. If you throw a ball one side of your body pulls, the other pushes creating a rotation. Same for shovelling, racking, pretty much anything.
When those muscles are unbalanced you have a strong muscle attempting to work with a weaker one, and it can’t keep up. This can lead to pulled muscles in your back, hamstring, it can cause knee injuries, etc.
We already live a fairly anterior chain dominated lifestyle. A lot of people spend pretty much their whole day using their glutes and hamstrings as something to sit on and little more.
So don’t neglect the posterior chain, it is vital for athletic performance as well as injury prevention. It might not be as easy to train, and the aesthetics of it might not be as important if you are just looking to look good, but it’s a vital part of proper training and getting the best results you can.