Billionaires is a Who Wants to Be a Millionaire-style quiz: a question with four possible answers, a prize ladder that doubles as you go, and the option to stop and keep your winnings at any point rather than risk them on the next question.
That choice is the game. The questions themselves are general knowledge of varying difficulty; the interesting decision is always whether the money you have is worth more than the chance at the next rung.
Yes — you can stop after any correct answer and keep your total. Knowing when to stop is the actual skill in this format.
The run ends, and you keep whatever the last safety level guaranteed rather than your full total.
Yes — tap an answer. Nothing to install.