An APK is a single install file. Treat it the way you would treat a parcel: confirm the address, the weight, the seal, and the sender. Four checks, in the same order every time.
APK mirrors are third-party hosts that re-publish install files outside the operator's own site. The four signals are mechanical: address, weight, seal, sender.
The mirror's address bar differs from the operator's published domain. A single character difference is enough.
The published APK size differs from the mirror's file size. A difference of even a few KB can indicate re-packaging.
The APK signature is published on the operator's download page. A different signature after install voids the install.
The mirror has no operator-published reference, no SSL certificate match, and no support contact. A genuine source will have all three.
Twelve boxes, ticked in order, before you tap install.