I haven't played slay the spire, but I've been enjoying monster train which is mostly the same game.
I would consider the core loop to be each encounter/game. This is a core loop that's shared across most card games whether that's gwent, hearthstone, magic, or Yu-Gi-Oh. It is the smallest self-contained unit of gameplay. The minimum viable loop would be each turn, but you can't really stop and pick up the game between turns, right?
All of the other loops, the build-play-build loop, the "finish a run" loop, the "cross-run-progress" loop, are all features that are enabled by/rely on the core loop of gameplay.
In analogy to tennis and the OP, each hit of the ball is the minimum viable loop, each point is the core loop. This is enhanced by the match loop and on top of that the "tournament" loop.
I would consider the core loop to be each encounter/game. This is a core loop that's shared across most card games whether that's gwent, hearthstone, magic, or Yu-Gi-Oh. It is the smallest self-contained unit of gameplay. The minimum viable loop would be each turn, but you can't really stop and pick up the game between turns, right?
All of the other loops, the build-play-build loop, the "finish a run" loop, the "cross-run-progress" loop, are all features that are enabled by/rely on the core loop of gameplay.
In analogy to tennis and the OP, each hit of the ball is the minimum viable loop, each point is the core loop. This is enhanced by the match loop and on top of that the "tournament" loop.