How to unlock nazjatar flight

i viewed a youtube vid and it says to explore some places such as tiragarde sound or stormsong valley. What does this mean? like revealing the map by going there or doing all the quests?

Unlocking flight in BfA requires completing Part 1 and 2 of the Pathfinder achievement. (It’s under the exploration section of the achievement interface.)

There are multiple parts for the achievement. One of them is to explore and reveal the map, and another one is to do all of the main quests in each of the zones (and the quests that tie them all together.) The best place to start is by doing the questing, as that will also help you work towards other parts of the achievement. (You’ll get most of the map explored by doing the quests, and you’ll also start working on the faction reps which is another requirement.)

Be aware that it is a time investment. (The reps require a fair amount of grinding.)

Here’s a Wowhead guide for the achievement:

is it possible to only unlock nazjatar and not do other parts for other areas?

Nope. BfA flying is all one unlock. (Shadowlands is actually the outlier in that regard with separate unlocks for the early zones vs. Zereth Mortis.)