Assassins could focus on traps and spells they D2R Items summon or martial arts abilities. Druids could be shapeshifters, or rely on elementsal attacks. The class you choose to play depends on it, you could also build your entire playstyle on a single skill for instance, Zeal Paladins or Blessed Hammer Paladins. They are widely known as Hammerdins.
Although you are free to mix and match skills in any way you want however, it's best to plan your strategy and then invest as per the plan. You have a limited number of points for each skill that means you won't successful in acquiring all of the skills on the tree of skills. And even if you do invest a lot of money in a certain ability.
This can determine which skills you should choose due to synergies in skills, which can be found by hovering over a skill. For example, the base Raise Skeleton ability for Necromancers receives a bonus from Skeleton Mastery and Summon Resist, which makes the skills crucial for summoners to select.
Resurrected comes with a respec option, so you aren't entirely locked into the choices you make, but there are limitations on it (you'll gain respec-related opportunities on a free basis as you play in the future, and subsequent ones require you to collect certain resources) buy diablo II resurrected items. Therefore, don't expect to be able to freely experiment to the same level as you could in Diablo.