Rename the .apk file extension to .zip, for example: Magisk-v22.0.apk → Magisk-v22.0.zip. If you have trouble renaming the file extension (like on Windows), use a file manager on Android or the one included in TWRP to rename the file.
Flash the zip just like any other ordinary flashable zip.

If you insist on using custom recoveries, rename the Magisk APK to uninstall.zip and flash it like any other ordinary flashable zip.