system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are MAC addresses generated for virtual or physical network adapters? Do they have to be globally unique and not reusable? If so, how is that guaranteed?

There are many types of network adapters, many different manufacturers, all on potentially huge internetworks. Adapters get removed, changed, and added all the time. Is a guarantee of ARP that the link layer address is unique? If so, how is that uniqueness ensured at that layer? If it is ensured by the nature of MAC addresses themselves, how are those addresses generated or issued so there is no chance of collision?
gpt
For physical devices, the manufacturers are allocated ranges of MAC addresses and are responsible for setting the default network address to be different for each device they manufacture.  It's possible to configure network devices to use a different MAC address, other than the default, and this is sometimes done so that routers can have multiple devices, each on a different network, all with the same MAC address.

For virtual interfaces of the kind used to allow multiple IP addresses on the same network, there's still only one hardware ethernet device and the MAC address is the same for each IP address.  Packets are allocated to the virtual interfaces according to their IP addresses.

You talk about "potentially huge internetworks" but MAC addresses only need to be unique on the same Ethernet network (at what is known as layer 2).  The Internet Protocol, with routers passing packets around the world, works above that at layer 3.