There are many ways to skin the cat for centralized login in Linux, including using Samba-AD or just LDAP.
Patching is IMO less fun. Landscape can work for Ubuntu but it’s finicky, and I haven’t really found anything satisfactory (FOSS) for patch management if multiple Debian systems. Setting up “unattended-upgrades” does tend to handle most of it but that doesn’t give centralized control or visibility.
A lot of people use automation systems etc. They work, but don’t provide the same GUI/reports you might see from RHS or Windows patching systems.
I too was surprised at how sparse or apparently kludged-together the pickings were.