I am using QGIS 3.16 and Windows 10.

I have some custom CRS (cCRS). Now, I'd like to work on my QGIS Projects at another PC and want to move my cCRS there.

According to the QGIS manual the cCRS are stored in the "user database". But I cannot find any further information about filename or path of the "user database" in the manual. I also tried to search for my cCRS-names in the Explorer (search for files including content). I searched in the (typical) user and programm directories, but haven't succeeded yet.

Does anybody have an idea for me?

In response to @JGH's answer I am including a picture of my folder contents: