When should I press the Apply button in modifiers (Blender 2.91) - in case I want to change the mesh as well? In object mode, I can already see all the changes without clicking Apply. Thanks! :)
Short answer: when you have to.
There could be all sorts of reasons that you want or need to apply a Modifier.
For instance a face is mostly symmetrical, so you will start modelling with a Mirror Modifier on. But at a certain point, you will want to slightly break the symmetry and add asymmetrical details, so then you would apply the Mirror Modifier.
Note that you will usually have to apply Modifier in the order they appear in the list, so if the Mirror comes after an other Modifier, you will have to apply that other one first (or you will get unexpected results.
I can't go through all the times that you will need to apply a Modifier (depends on the Modifier and on the situation), but generally speaking, I'd say: don't apply a Modifier, unless you have to, and you will know when it is necessary.