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Quote from the minecraft wiki:

"Once activated, TNT turns into an entity. It is spawned at the center (+0.5,+0.5,+0.5) of a powered TNT block, while itself is a cube with an edge length of 0.98. Its fuse lasts 40 redstone ticks (4 seconds/80 game ticks) if activated by redstone or fire, or a random number between 10 to 30 game ticks (0.5 to 1.5 seconds) if it's destroyed by an explosion."

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TNT will be calculated every game tick (20 times a second). If it is in motion it will fly in the y direction then x then z. This means the TNT will fly like a stair.

On every tick the TNT will be fully calculated one after the other. First is the motion calculates then the explosion if the TNT is 4 seconds old (Fuse:1). Thats why multiple TNT exploding at the same time can dig inside a wall if the motion carries in one tick. You can also make a TNT cannon where the propellant the projectiles are ignited at the same tick.