What happens if the only DC fails?

I recently started an internship in System Administration and I am currently tasked with researching and potentially setting up an on Premise Active Directory in our company. Lets say, I am running Windows Server 2019 or 2022 on a single DC and using AD, ADDS etc. services. Using the Standard Edition with 16 Core licensing and 40 User CALs.

My first question is, what happens when the only DC fails/becomes unavailable. Which services would still function, if any.

If I understood correctly, if I want to add a second DC, I need another 16 Core License, because you need 16 Core License minimum per server, correct?

Which brings me to my second question. Can I run a second DC on a VM? Either on one of the 2 VMs included in a Standard 16 Core License or entirely on a different VM.

I am still fairly new to this, so any help is greatly appreciated!

Can I run a second DC on a VM?

In cluster environments, people run both DCs on VMs but store them locally on different hosts. So, it is fine to run DC on a VM.

It's standard practice since maybe Windows Server 2012 to have a) virtualized domain controllers (DCs), and b) have them configured in highly available (HA) mode. DCs have their own replication mechanism built-in, so this comes at no extra management cost.

Microsoft's recommendations on virtual domain controllers:

https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/get-started/virtual-dc/virtualized-domain-controllers-hyper-v

AD HA (incl. Domain Controllers):

https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/get-started/replication/active-directory-replication-concepts

Hope this helped :)

This is more than one question and a bit out of scope of sf. But here is quick answer anyway.

what happens when the only DC fails/becomes unavailable. Which services would still function, if any.

All services depending on th AD will fail, everything else will work. As you did not post any services nor their configuration, we cannot answer this.

But it is strongly recommended to have at least a second DC. Usually the first thing that you will notice in a fail case is that your users are not able to login.

if I want to add a second DC, I need another 16 Core License, because you need 16 Core License minimum per server, correct?

Windows Server Standard, as of today, can be installed two times (on the same hardware). The standard license comes with two instances, which is why virtualization makes eveything easier.

Can I run a second DC on a VM?

Sure - and you definately should.