The functioning of taxis in Nepal is quite the same as in the western countries. Indeed, you will be able to present yourself in front of any cab so that it can take you where you wish. Most of the time, they see if you need a cab or not and come directly to you. However, if you are a tourist, they will charge you more in terms of transportation costs so try not to get too caught up. Example: a trip from the airport to the center of Kathmandu (Thamel district) should cost you about 400 Nepalese rupees for a 30min ride.
Do not hesitate to use local applications such as "Indrive" or "Pathao" which will allow you to order a cab directly via your phone and especially not to pay excessive rates.