How does the density of a water in a tank and the density of a water in a glass can be same density since water's density is 1
and it's formula is mass/volume
since the tank's water is more than as a volume that of the glass and also more than as a mass that of a glass

Just because there is more mass in a certain substance doesn’t mean it’s density will change. In this case, water, will not change density simply because there is more of it. I.e the ocean. The bottom of the ocean may have more pressure to it because of the weight of the water above it but if you were to take a cup of it and take it to the surface it wouldn’t sink.