You have wire the statues to a timer or switch… also you have to do damage to the mob it can’t be done by lava like a normal farm… also most of the mobs have a reduced drop rate and or restrictions… except for slimes and I believe 2 others but I can’t remember which… this is all I know as I don’t usually make statue based farms-nyah.
You also might be able to use boulder statues for building a farm as well since they spawn boulders but that’s not something I’ve personally tried-nyah.