Crystal Connect is a pair-matching puzzle with a path constraint: two identical crystals can only be linked if the route between them is short and unobstructed, so an obvious-looking pair is often unmatchable until you've cleared what's between them.
It has a fantasy framing — dwarves collecting crystals — but the puzzle underneath is pure connection-matching, and the order you clear in decides whether the board opens up or locks.
Because the path between them is too long or blocked by other crystals. Clearing pieces in between usually opens the route.
Levels typically have a time element, with successful matches extending it, so a steady run of clears sustains itself.
Yes — tap two matching crystals. Nothing to install.