Think of the Netflix "Ta-dum" or the Apple Mac startup chime. These are SFX that have become iconic brand assets. Categories of Sound Effects to Download
SFX should support the story, not distract from it. Ensure your "hits" aren't clipping (distorting) and that background ambience doesn't drown out dialogue.
High-impact sounds like risers, hits, drops, and "braams" used to build tension in trailers and action sequences.
A massive database of user-uploaded sounds. Always check the specific Creative Commons license (some require attribution).