I am going to assume that you have a concrete patio which has a small gap where it meets the house foundation.
Seal gap between siding concrete.
How to seal and fix a gap between foundation and concrete sidewalk or patio.
Always caulk between the siding and the trim.
I believe you have tar paper under your siding based on your description.
Do not caulk between siding and any built in receiving channels located at or around windows.
Use a small.
Crawl around the exterior of your home and use a flashlight to find cracks holes and gaps.
Find the entrances mice use to enter your home.
Note that there are various types of caulk.
Since some of the siding looks like it will need re priming and painting make sure that you use a paintable type of caulk.
Loctite pl10 often the gap to be filled is quite deep.
Vertical trim gap caulking.
How to seal siding to keep out mice.
All it takes is backer rod and concrete flex sealant.
In that case the use of backer rod before caulking is recommended.
They are typically mechanically robust adhere extremely well to almost everything especially wood and concrete and are stretchy enough for seasonal building movement.
The best solution here may be to clean up that area of all debris let it be fully dry and then apply a bead of qood quality exterior grade caulking material.
Theoretically you could seal the tar paper to the foundation with foam spray caulking something similar.
Leave a 1 8 gap between the siding and the trim or other materials to allow for structural movement.
Look under the siding at the area meeting the foundation of your home for holes and gaps.
In this case what i would use is a polyurethane caulk made for masonry like this one.