Why is acceleration taken to be -g in vertical projection even when the object is falling towards the ground

acceleration is -g when the object is going upwards but why is it still -g when the object is reaching the ground? Isn't the object accelerating in the same direction as gravity?

I only need to know what direction to consider in projectile motion.? - because sometime a projectile going up and then down or its going from up to down

If the acceleration and velocity have the same sign (both + or both -) the object speeds up. That's what happening when the object falls downwards (v and g are both negative).

If the signs are opposite, the object slows down.

Choosing positive for upwards is just a convention, you could choose negative for upwards. But you have to be totally consistent within the calculation.

In these sort of questions, the velocity sign is changing as well. When the velocity sign changes, the constant negative acceleration will act to increase the velocity instead. This is why you don't change the sign of gravitational acceleration - the change in the sign for velocity takes the change in direction into account already.

i never use gravity as negative when a object accelerates to earth. It confuses me, but the reason i was told was because apparently loads of people get the final answer confused in exams ect. But i get more confused when i use it as a negative when it travels to earth.

When something is thrown in an upwards direction the velocity is positive (+) =g

when something is thrown downwards the velocity is considerd (-) because it is going against gravity.

We assume that the up direction is positive. Nothing stopping you from defining down as positive as long as you remember to change the signs appropriately.