Can someone ELI5 how you “weigh” the earth? Isn’t weight based off gravity? And gravity is relative to the mass of the object you are on, for example we say you

Can someone ELI5 how you “weigh” the earth? Isn’t weight based off gravity? And gravity is relative to the mass of the object you are on, for example we say you would weigh less on the moon and more on Jupiter. So how exactly to you weigh a planet, and why would that even be useful? Do you assume some theoretical force and hypothetically place the earth on a scale and apply that force? When talking about planets isn’t mass a better way to measure and classify rather than weight?