It can depend on the type of UI you are looking for but generally websites like Screenlane, Godly, Mobbin, Design spells and pttrns.
and of course there are always options like Dribbble or Behance there.
In case you are looking for fancy designs, Dribbble and Behance are good options.
In case your are looking for real life examples - Mobile Patterns, Landingfolio, Refro, Design Vault are good options in addition to the ones mentioned by @tarane_yari
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