long story short it had to be "an app called tiktok".

i think people often use "a/an" instead of "the" for something new to the reader.

✅ I've just learned (that) there's a thing called an MBCG.
("a thing called" is "to iu mono" in jpn)

when i was a child, i picked up a cd-rom of a 40 something woman's photo book.
it was just swimsuits.
i felt it was very valuable—she was very famous in my world.

couldn't sleep at all bc i went to sleep at 7am yesterday (it's 4am now)

gonna sleep for a bit

The app called TikTok should be banned. ❌️
An app called TikTok should be banned. ✅️

in this context "an app" would've been more natural. i got it wrong 2 years ago.

it's grammatically correct but unidiomatic (except in Irish English etc.).
seems you can also say:
I've got a book. ✅️