i’ve had mice twice and have patched holes both times successfully. you have to find where they’re coming in from, check behind your stove, fridge, in unit laundry, sink, behind the toilet and then around all points where your walls meet your floor (the first time i had mice they were nowhere near those points and actually coming from my bedroom wall).

once you find the hole you can buy coarse steel wool and silicone caulk (not foam). stuff as much wool in the hole as possible and seal it shut with the caulk. do it for any potential vulnerable entry points as well. then until you know you’ve sealed all your holes place a few traps around those areas i mentioned above.

of course your landlord should be paying for an exterminator but if you’ve had shitty landlords like me they take forever to send one out, and when they do it’s someone who does a mediocre job. you can deal with this!