THHN wire is good for service entrance wire as long as it's placed in conduit.
For a service entrance wire to your electric meter box it should be run conduit and if you have overhead wire then it should also have a weather head that the service entrance wire comes out of.
For copper service entrance wire you need #3 copper wire for 100 amps service but you can usually get away with #4 copper wire if you're not using 100 amps constantly.
#4 copper wire is rated for 85 amps which most homes don't even draw that much power at the same time so it's usually okay.
Service entrance wire is the wire that leads from the weather head to your electric meter.
The service entrance wire also leads from your electric meter to your breaker panel indoors.
Then from your breaker panel the regular copper Romex wire distributes the electricity through your house or building.
Service entrance wire can be either copper wire or aluminum wire.
The copper service entrance wire is a smaller wire size than the aluminum service entrance wire is.
Aluminum service entrance wire is usually a 2-0 wire for 100 amp electric service.
For 100 amp electric service for copper wire the service entrance wire is a # 4 AWG size wire.
Copper is a much better conductor of electricity and it's less prone to corrode than aluminum wire is.