Even when Combat and Magic are also Skill best there are often have extra steps like using mana points or spell slots, and of course damage rolls and HP. So they not pure Skill use, in the manner of picking locks or investigate
Combat - Magic - Skill are thus the three mechanical systems used in RPG games to do things, they often overlap even to the point of all being skill-based, but there is often enough to distinguish the three
In games that have both skills and some version of advantages/perks/edges, both combat, magic, and "other" often use a mix of the two, though the mix is often different depending on your focus. But that's a matter of degree, not a strict delineation.
For example, a fighter-type in Savage Worlds needs the skill of Fighting and/or Shooting, as well as a decent Strength (for dealing damage – even weapons that aren't directly muscle-powered often have Strength minimums) and Vigor (to sustain damage). Agility is also nice because it makes it easier to raise the combat skills in question, and of course there are other skills that are useful but not core (e.g. Athletics, Notice, and so on). But much of the stuff differentiating one fighter-type from another is a matter of choosing Edges.
An infiltrator-type, on the other hand, is more skill-based: Athletics, Stealth, Notice, and Thievery are a must, Persuasion is good to have (as it also covers deception) and depending on the setting you might want things like Electronics or Hacking. There are Edges available to improve infiltration, like Thief or Acrobat, but nowhere near as many as the variety you have with different fighting styles. And of course, there are many other variants of primarily skill-based characters: investigators, faces, medics, and so on.
And finally, arcane power users are also more Edge-based. Each type of arcane power has its own Edge to unlock it, and its own skill (e.g. Spellcasting, Faith, Weird Science). There are some generic arcane Edges that are used for all sorts of "casters" (notably New Powers to increase your repertoire and Power Points to get more PP with which to activate powers).
But there's nothing preventing you from mixing and matching between these, other than available resources (every advance spent on New Powers is one not spent on increasing your skills). A "thief" can probably get away with a high Shooting or Fighting skill, but they probably don't have the resources to have
both good Shooting/Fighting, good infiltration skills, and a bunch of combat-related Edges. So they won't be as useful in combat as a dedicated combatant, but they won't be useless.