The file has to be reachable, then it can go
Most networks do not take the file from your browser. They download it from an address they can open.
Who downloads it
TikTok, Instagram and Reddit fetch the URL after QuickPost has already returned. YouTube and LinkedIn are different: QuickPost downloads the file and uploads the bytes. Either way the address has to be public https while the send is in progress.
After a full publish
A file you uploaded through Compose is removed about two hours after every selected account has published. That wait is there so a network can finish its own download. A link you pasted yourself is left alone.
If one network fails
The file stays while any target is still failed or waiting. Cancelling or deleting the last post that uses it removes the file immediately.