How to choose the right dumbbell/bar weight for training?

Weights are selected in such a way that during the exercise, there is a sensation in the target muscle groups, the performance is correct, total control over the body and/or the equipment is maintained, and the pace and the number of repetitions or time range under load (if the exercise is for time) in all approaches/series/circles of the particular exercise is respected. 

As soon as you manage to perform all set reps and sets in an exercise, add 5 kg (total) in the last set for large muscle groups (legs, back, chest, partially shoulders) in multi-joint/basic exercises. For smaller muscle groups (triceps, biceps, calves, forearms, abs), add 2.5 kg (total). 

If you manage to do the necessary number of reps or keep the set time interval under load, then add the same weight in the penultimate set.