You have siding overlapping to shield the house and direct water away. It seems like judging from the picture that the piece is so close to the overhang that the place will be free of water and therefore I would ignore it. If that is not true I would get a wider piece and overlap it with the first real siding piece.
You can buy rolls of 'bottle or bristle brush' type material in different diameters. Essentially plastic bristles with a twisted wire core, simply cut to length and pushed up into place.
Originally designed to deter mice, but it'd work for crickets just as well.
That is 99.456% likely to be a plastic house wrapping such as Tyvek found in this picture:
DO NOT seal the bottom with any sort of impermeable caulking nor spray foam. The bottom should remain permeable so that if water gets behind your siding for whatever reason then ...
It looks like there is a layer of foam between the siding and the house this was quite common.
I would not seal below the siding, this could trap moisture and since there is a layer of foam attached to the sheeting there really shouldn’t be any air movement and this is outside the structure, the stud bays them selves are another air space beyond that.