It's actually pretty simple, just confusing in presentation.
The Path of Champions cares about:
1. Legend Points (your path-wide XP to unlock stuff, kind of like your 'account level')
2. Fragments for Starring Up champions
3. Relics to equip to champions
4. Levelling up champions by playing with them (upgrades their deck and unlocks better relic slots)
That's really it. You can level up champions by playing any mission with them, more difficult gives more XP. Fragments, Relics, and Legend Points come from completing quests or adventures for the first time (not repeated grinding).
Doing your "Chapter Quests", Daily & Weekly Quests, and making sure you complete each adventure's requirements (early ones only need you to beat them once with a single champion, but later ones require beating them with multiple champions - often of specific regions, as described int he adventure details when you tap the adventure) will pretty much handle eve...
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