Our own certificate authority issues the certificates our internal sites and applications use. The root certificate is public: copying it grants nothing, which is why it can be published. With it installed, our internal sites stop raising warnings.

Before installing any root certificate, check its fingerprint with a trusted person — not against the page that served the file, which cannot vouch for itself. Ours carries name constraints: it is limited to our own names, so a certificate it signs for any other name is refused by your machine.
SHA-256  51:D3:71:FC:1B:DC:B8:44:9F:A4:92:9B:3D:24:46:2A:6D:0C:FA:0F:CC:B3:4E:EF:61:81:1D:98:3A:62:12:62

Download the root certificate

How to install it on each kind of device, and how to remove the earlier root, is on help.ploutos.xyz.

How this fits into sign-in and access is under Identity & access.