It's both.
Let me paste the equations I didn't post them on itch yet.
The efficiency bonuses from stats multiply the costs by 1 / (1 + StatLevel / 50)
The XP bonus from permanent levels multiplies XP gain by 1 + ((Level ^ 0.8) / 5)
Most skills multiply costs by 1 / (1 + SkillLevel / 25)
MNST, FISH, and BIRD multiply the cost of removing rocks by 1 / (1 + Level*ResourceAmount / 100)
The STAT skill adds a flat 4% of your permanent level to your current level for each STAT level.
By default you get 100 skill XP per learn. The XP cost starts at 30-90 and goes up by the original cost every level. LERN gives 4 extra points per level.