You're seeing just the callback lock_for_migrations, go back to Ecto.Adapter.Migration module for more information.
Link to this callback

lock_for_migrations(adapter_meta, options, fun)

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lock_for_migrations(adapter_meta(), options :: Keyword.t(), fun) :: result
when [fun: (() -> result), result: var]

Locks the migrations table and emits the locked versions for callback execution.

It returns the result of calling the given function with a list of versions.