It can be anything! An apex, subdomain, or even a URL.

Results forauth.firebase-dev.sidelinehd.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2021-03-27T10:00:21.547	2021-03-24T23:27:26	2021-06-22T23:27:26	www.cybergoose.pieterjanderidder.be	auth.firebase-dev.sidelinehd.com	C=US, O=Google Trust Services, CN=GTS CA 1D2
2021-03-25T00:27:27.284	2021-03-24T23:27:26	2021-06-22T23:27:26	www.cybergoose.pieterjanderidder.be	auth.firebase-dev.sidelinehd.com	C=US, O=Google Trust Services, CN=GTS CA 1D2
2021-03-24T22:19:56.185	2021-03-24T21:19:55	2021-06-22T21:19:55	auth.firebase-dev.sidelinehd.com	auth.firebase-dev.sidelinehd.com	C=US, O=Let's Encrypt, CN=R3
2021-03-24T22:19:55.932	2021-03-24T21:19:55	2021-06-22T21:19:55	auth.firebase-dev.sidelinehd.com	auth.firebase-dev.sidelinehd.com	C=US, O=Let's Encrypt, CN=R3