In order to train for functional strength, you should focus on training movements, not muscles. Rarely does the body only use one muscle at a time. They work in concert with each other.
What are some basic movements? How about push, pull, twist, bend and squat. Pushing can occur in front of you or overhead, like with a push-up or military press. Pulling can actually occur a few ways, like in pull-ups, rows or cleans.
I like the deadlift. It’s very functional since I pick up things from the floor to carry all the time. You get a squat and bend motion in there. Twisting and bending work the core and can be done with core work or even loading a truck with sandbags.
During these movements all the smaller muscle get worked out too. Who needs to do forearm wrist curls if they do heavy deadlifts? In my opinion, nobody. Holding onto that heavy bar will work your forearms plenty. Same thing with doing pull-ups.
Here’s a quick video I found explaining training movements.