PopStar is a tap-to-clear block puzzle: any group of two or more connected same-coloured blocks can be removed with a tap, and everything above falls down to fill the space.
The scoring is quadratic — clearing ten blocks at once is worth far more than five clears of two — so the whole game is about growing groups before popping them rather than clearing whatever's available.
It rises sharply with group size, so one clear of ten blocks is worth far more than five clears of two. Building groups before popping them is the whole strategy.
Yes — emptying the board entirely awards a substantial bonus, which is often what makes a level's target reachable.
Yes — tap to clear. Nothing to install.