After watching the video I just made I realized I may have implied that using DHCP instead of manually configured addressing is what makes the virtual machines connect to each other or not. That's not it. Switching to DHCP was just an easy way to change the addresses. I could have changed from 192.168.0 to 192.168.1 manually and it would have worked as well as using DHCP to do it.
I've used it about 12 years now - it's a great free hypervisor solution for a Windows workstation for general R&D.
I use Linux KVM for heavy lifting, or VMware otherwise. I'll use cloud servers as well depending. I'll run all kinds of OS's on it to do R&D stuff for work.
I installed FreeBSD on it tonight actually (I often run Debian, Ubuntu, Red Hat, CentOS, Rocky, VyOS and Windows servers and workstations. Even have a few MsDOS and FreeDOS systems.
So on your networking video - yeah pretty much the Internal Network option basically creates a dumb switch where the machines are connected to each other. It's a self contained environment. Each one you create is isolated.
You can use the "NAT network" option and it will allow you to NAT out to your home network (and can go outbound to the internet). It will allow related and established traffic back in (ie initiated from the inside), but not traffic that was initiated from the outside.
The other option is Bridged, which puts you right on your actual host network (usually your native home network).
Cool use case: You can create a VM in bridged mode (say a Linux Webserver) then have your home router NAT port 80 from the Internet right to it - then register a DNS name and use Dynamic DNS on your home router - and boom - you now have a www name on the internet that points right to the webserver on your Oracle Virtual Box linux VM. Anyways, very cool and low cost - leverage than unlimited internet plan. I did this over the years to host websites and game servers etc.
On your video, you'll find that if your two machines are on the same Internal network, but on different subnets, they will not talk to each other without a router between them (like 192.168.1.10/24 and 192.168.2.10/24).
A good free router solution is VyOS. Download the iso file and create a linux type vm and install it on that. You can then put two virtual nics on the vyos vm so it has a leg on both subnets, and it will act as a router. You can do some extremely complex networking with vyos - I've built scale models of complex networks to come up with working designs for customers (like 10 VyOs routers and 5 or 6 servers). If I have to build much more than that, I go to KVM on a beefy native Linux server as performs much better with a lot of VMs.
A setup like that is very useful for learning complex dynamic routing, encrypted tunnels & complex firewall work (plus a hundred other things).