The variety of options means each class can function slightly differently depending on your choice. With the Necromancer You could focus solely on summoning an army of minions. You might choose to create D2R Items character which invests points into poison/bone spells, which allows you to deal damage more directly.
Assassins can be focused on traps that they summon or martial arts skills. Druids could be shapeshifters, or rely on the elemental attack. The class you choose to play depends on it, you can also center your entire character around a single talent, like Zeal Paladins or Blessed Hammer Paladins, who are popularly referred to as Hammerdins.
Although you are free to mix and match abilities as you wish It is recommended to create a plan and invest appropriately. You have a limited number of skill points to allot which means you won't be capable of acquiring everything on the skill tree. Even if you invest heavily in a particular ability.
This could determine which other skills you choose due to skill synergies, which you can see listed by hovering over a skill. For example, the base Raise Skeleton ability for Necromancers has a benefit from Skeleton Mastery and Summon Resist, making the skills crucial for summoners to select.
Resurrected includes a respec feature as well, which means that you're not bound by the choices you make, however there are restrictions (you'll get some respec chances in free play each one, and later ones require gathering certain resources). This means that you shouldn't think of being able to freely experiment to buy diablo 2 resurrected items the degree that you can in Diablo.