qt5core.dll
This file is not a system component: the qt framework, shipped inside the application. Restore it from the source that installed it rather than downloading a replacement.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
What it is
The core library of the Qt application framework. Programs written in Qt carry their own Qt libraries in their install folder rather than sharing a system-wide copy.
Where you see it: VirtualBox, VLC builds, CAD and engineering tools, cryptocurrency wallets, and a great deal of cross-platform desktop software.
Qt versions are strict about matching: the application, Qt5Core.dll and every other Qt DLL beside it have to come from the same build. That makes downloading a loose copy worse than useless, because a mismatched one loads and then fails somewhere unpredictable. Reinstall the application. If the error appeared after you uninstalled a different Qt program, that uninstaller most likely removed a shared folder it did not own.
Why there is nothing to download here
There is no verified download we can offer for this one, because installing a file is not the fix. The steps above are what actually resolves it. That distinction matters: the sites ranking for this file name are selling a download that would not have helped even if it were safe.