People cooking it up domestically? Its not hard to make (if you don't mind killing some people) and its ingredients can be commonly found in the US.

Through small parcels from China mixed in with regular mail? People mail weed all the time and that's also illegal to go through the mail. A small parcel can carry many thousands of doses with high enough purity.

Most fentanyl in the US probably has little to no connection to Venezuela. Probably lots of other drugs, sure, but not fentanyl.

A lethal amount of fentanyl can be as small as 1-2mg. So enough fentanyl to kill 100k people would be like 50ish grams. Its not nearly some massive amount of material like you seem to think it is.