You really think USADA are flying all over the world testing fighters?
Its pretty simple, USADA are in a big connected computer system with every other national anti doping agency in the world. UKAD (UK), AFLD (France), CCES (Canada), RUSADA (Russia), etc etc etc
If they want a fighter testing, they raise it in the system, if the fighters location is in France, then AFLD conduct the test, if hes in Ireland, Sport Ireland conduct the test, Finland, Finnish anti doping, Dagestan, Rusada, etc etc
Its really not very complicated.
As for location. Fighters have to submit (to the very same computer system)
1) Overnight location
2) Normal place of training and hours they are normally there
3) Normal place of work and hours they are usually there
(Contrary to what people on sherdog tell you you dont have to give them your location 24/7, only those three things. Overnight location, place you train, place you work)
Generally testers will just turn up very early or very late at your overnight location.
If you arent there and cant be found, missed test, three in a year, ban.