There are several approaches to this training and you’ll want to take advantage of all of them:
1) Practice any of the foundation behaviors (nose touch, it’s your choice, tug, fetch…) in the presence of other things that are interesting (new environments, new people, dogs, noises etc.) See also “novel things are fun”
2) Teach the dog to “Get” whatever you are presenting as a reward even in the presence of a distraction (such as another reward) - in this case the other reward represents anything the dog might be distracted by. One method is to present a tug toy, with treats in the other hand, and encourage the dog to “Get it” (toy), OR use two toys and switch back and forth, OR use kibble and steak….
3) Set up specific categories of distractions so that you can practice reinforcing your dog for ignoring them. For example, when working on startling distractions, you might include smoke bombs, firecrackers, flares, sparklers etc.
1) Different dogs will find different categories of items distracting. Some dogs will find birds highly interesting - so you’ll want to work specifically on that. Some dogs could care less about birds, but the noise and movement of small kids is a dilemma. Breed and temperament can be clues, but don’t tell the whole story as every dog is unique.
2) Also dogs are learning all the time, they can learn not to be distracted and learn how to be as well - for example, be cautious of allowing your dog to sniff around the agility field when they are “off duty” in the thinking that they will learn to ignore smells when they are working.