I dont understood the formula P=Pa+pgh and its derivation

Consider a bottle of radius ‘r’ containing water upto height ‘h’.

The volume of water contained is, V = πr²h

The mass of water is = ρV = ρr²h

The weight of water is = ρπr²hg

So, pressure on the base of the bottle only due to water is = ρπr²hg/ πr²

Now, over the water the air is also exerting pressure. This is atmospheric pressure. This atmospheric pressure also adds up to give us the net pressure on the base which is,

P = Pa + ρgh

Where, Pa is the atmospheric pressure.