Glaxay S9 - e version confusion

yeah so I’m familiar with the “stock” flashing instructions. my main problem happens when the procedure deviates from there. i don’t exactly recall what the problem was but I couldn’t sideload TWRP last I tried… i got the OEM unlock, but it would complain about the signature being wrong.

i suspect that recent phones and tablets have protections about that kind of stuff, which is why i’m asking.