Usually it's merely /setblock on somewhere behind the walls.
The correct digits sets a stone next to a repeater. The wrong one removes another.
A total of 9 slots (aka 9 repeaters in a chain, with a gap between each) will be placed, if they got the correct order, the signal will go through all 9 repeaters and blocks and reach the command block.
If you're doing permutations (it might be different, but I mean combinations that match the exact order) tho, that's another story...
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