At its very base, the game has levels and job levels, but they all interact with cards in a somewhat bizarre way.
Your character's level = the level of your 4 equipped cards. This affects your base stats like Attack, HP, etc.
Your 'job level' = how much shit you unlocked on a Job card's specific skill panels. the "XP" for these are Skillseeds, which you get after each combat based on the cards you have equipped, multiplied by a few factors (area, how efficiently you kill stuff, p2w bonus, etc)
Your cards - As mentioned, their levels feed your player level and their skillseed unlocks feed how fast you level your jobs. The cards themselves also deal damage/effectiveness based on their ability level. A lv6 Ability is far, far more effective than a lv1 Ability.
In other words, all 3 do measurably add to power level, and to a certain extent you can't really tunnel one without inadvertently working on the other two anyway.
In terms of what cards to work towards (in my very short experience with the game - didn't even get this far on JP launch):
Ideally you want to be working towards a way to nuke stuff very fast, which involves pumping a card's ability level, and then making sure you have the support necessary to feed it orbs. For example, right now I'm working towards maxing out a Blizzara card (since my job class is mage and also gives extra water damage %) and a support that shifts all orbs to Ice.
There's generally 3 types of targeting - single target, cone, and AOE. I'm finding cones to give the most bang for the buck. AOE is too cumbersome in terms of orb cost and single target doesn't wipe out a lot of trash waves fast enough.
edit: Card Evolutions unlock after progressing through the main story but they're straightforward - there's evolution materials for each element for evolving up to 3*, conveniently dropped by the bosses on the map (denoted by the colored Bomb icons) and 4* evolutions are paywalled/timewalled by "Growth Stars".
I believe generally shop cards can evolve beyond 3* and drop cards can evolve only up to 3* but are important for feeding Ability levels to shop cards via fusion (if you're not using them outright)
edit2: Also progressing the main story uncaps the maximum levels of cards. Yes, it's bizarre ehehehe