the entrance to stratum 5 is in the outer forest bottom left area with a golden platform, you need to atleast have seen stratum 3 incident and blanche getting imprisoned there to access the sidequest
and to access that area of b1 you need to fall in the right place in cave f1 since stepping on the cracks create holes
refer to this guide https://docs.google.com/document/d/1zCvffgFXAzjnDPx5-GLzhftlsegzke-trCLgwDqaeUw/...