Why is этото used when the noun (мыши)is plural? Shouldn't it be эти? I've noticed this with ета етот and ети for other sentences too and it's really confusing can someone please explain
You mean это right? When это acts like an adjective (this car, these mice, etc) then it must match the thing it's describing in number, gender, case. But when это is a standalone pronoun not describing something, (as in the situations it is, this is, these are), then it is always just это. The verb will change for plurals but not это.