On the Samsung Galaxy A20 (and most devices running Android 9 Pie and above), the vbmeta partition acts as the gatekeeper. If you try to flash a custom recovery like TWRP or a patched boot image for Magisk without dealing with vbmeta , the gatekeeper will see a mismatch and stop the process.
image contains cryptographic signatures and hashes for other partitions like NXP Semiconductors Verification: vbmeta samsung a20