When a validator adds a block to the ledger it can include evidence of slashable offenses. Including evidence of a slashable offense gets rewarded so every validator is incentivized to include them.

Further reading: https://eth2book.info/altair/part2/incentives/slashing#the-v...

There is no separate notion of a slasher, everyone gets to provide slashing evidence when they build a block. I don't understand why you build such a weak straw-man and the push it forward as if it was the truth without even checking how the system operates.