Is the MAC address sent anywhere for the lookup?

No. The address is analysed entirely in your browser and no network request is made, so the MAC is never transmitted. Vendor names come from a small built-in list, not a live query.

How are unicast, multicast, universal and local decided?

They come directly from the first octet's two lowest bits, as defined by IEEE 802. Bit 0 is the I/G bit (0 = unicast, 1 = multicast) and bit 1 is the U/L bit (0 = universally administered, 1 = locally administered). This is fully deterministic.

Why does my phone's Wi-Fi MAC show an unknown vendor?

Modern phones use randomised, locally-administered MAC addresses for Wi-Fi privacy. These have the U/L bit set and intentionally do not map to a real manufacturer, so the vendor will show as not in the local list.

MAC Address Lookup — Gera Tools

MAC address lookup

Decode any EUI-48 MAC address offline. The tool normalises the address, extracts the 3-octet OUI (Organisationally Unique Identifier) and reports the vendor when it is in the built-in list.

It also reads the two IEEE 802 control bits in the first octet: the I/G bit (unicast vs multicast) and the U/L bit (universally vs locally administered). Locally-administered addresses are typically randomised privacy MACs used by phones and virtual machines.

Everything is computed in your browser — no lookup request is made, so the address is never sent anywhere. For a manufacturer not in the local list, consult the official IEEE OUI registry.