Almost never, and almost certainly not in the cases you’re thinking of.
On 32 bit platforms, Int refers to Int32. On 64 bit platforms, Int refers to Int64. There is no equivalent shorthand for floating point numbers.
For variables, type assertions limit the type of that binding in the current scope to the asserted type.
For function arguments in function definitions, type assertions restrict the allowed incoming types for that argument.
For struct fields, type assertions signify that the field in question holds a value of the asserted type. The default constructors will try to convert incoming arguments to the types in question.
For more information, see the section on type declarations in the docs.