Pareto's principle of fitness.

Paretos's principle states that for many outcomes, roughly 80% of consequences come from 20% of causes.
80% of sales come from 20% of clients
80% of wealth is held by 20% of the population
80% of sporting wins tend to be generated by 20% of the players.

Now that I'm training in a commercial gym again, I notice quite often that I arrive while some of the members are doing bicep curls and calf raises, and inner thigh machines and single arm, half kneeling, offset cable tricep kickbacks....and by the time I've finished the bulk of my session, with just enough time to hit a couple of cheeky sets of biceps or shoulders, they are STILL doing similar movements.

I'm not anti any of these movements, it's actually been a load of fun to play around with some new toys. But I don't neglect the basics.

Pareto's principle holds true with fitness as well.
80% of your fitness gains come from about 20% of possible exercises, namely compound (multi-joint) and functional (free weight) ones.
Want bigger biceps?
Pullup, row, rope climb and THEN do a few cable preacher curls

Want to peachify that booty?
Squat, lunge, deadlift, step up, romanian deadlift, sprint, sled push, box jump and THEN do a few glute band hip thrusts

Want to get a six pack?
Do all of the above strength training exercise and more, 3-5 x week, walk 10000+ steps per day, eat protein and veggies at every meal, sleep 8 hours a night, and THEN do some Kayla Itsines ab exercises.