I believe you have to remove the trim and seals from inside then remove window realign outer seal
replace window this should compress outer seal when inner seal and trinm is put back.
It is designed so the seal apply equal pressure to the galss prevents shock etc. and provides watertight integrity.
I dont understand how your seals are falling out? unless they have always been like tis? it is very difficult to remove outer seal first. They dont just fall out?
If seal is not there window would be loose and not waterproof.
Should be able to buy seal by the mtr at hardware or window shop.
You could if you think it needs replacing cut a small piece off and take it with you.