Unlike official games like Dragon Ball FighterZ or Naruto: Ultimate Ninja Storm , Anime Mugen has no rules. Want to see Sailor Moon fight Eren Yeager? It’s possible.
M.U.G.E.N is a free engine. If a site asks for money, it is likely a scam.
Go to your device's settings > security > and enable "Unknown Sources" to allow installation of apps from outside the Google Play Store.