You can check the first few digits of the MAC address at https://standards-oui.ieee.org/oui/oui.txt

That'll give you the vendor of the device's NIC.

If you have access to the ARP table on your router, you can translate the MAC address to an IP address. Often with arp -a or show arp on a router commandline.

On your PC, you can check hostname of the corresponding IP and whois information:

Check if you can find out the current connections to your router with a netstat-like command. Check IANA's Service Name and Transport Protocol Port Number Registry what the port is commonly used for.

This should give you a handful of leads to further analyse.