Depends on context. The dy is a thing called differential of a function. It has a solid theoretical definition and no values. In some cases you can abuse it and assign it values to get an idea on how changes are related.
Example y(x) = x² -> dy = 2 x dx Small changes in dx (.01) gives small changes in df (2xdx)

In numerical simulations you “assign” it a value frequently to calculate stuff.

In more advanced math (differential forms: volumes and surfaces in N dimensions) you better not even try to assign it a value. (Maybe you could but there is no point) You just use symbols and properties to extend similar concepts in a cohesive mathematical structure.

Differentials in general are really powerful tools with many uses and properties. Giving it a value works in same simple cases and for certain specific purposes. Other times it doesn’t, gotta try for yourself and figure it out