What is the number of significant digits for a number that does not represent a measurement?

The respective number of significant figures for the numbers $23.023$,$0.0003$,and $2.1 \times 10^3$ are:

The radius of a thin wire is $0.16 \; mm$. The area of cross-section taking significant figures into consideration in square millimeters is?

The number of significant figures in $4.870 \ m$ is

From the point of view of significant figures,which of the following is/are correct?
$(i)\ 11.3 \ cm + 4 \ cm = 15.3 \ cm$
$(ii)\ 4.53 \ m - 1.2 \ m = 3.3 \ m$
$(iii)\ 5.45 \ kg - 3.2 \ kg = 2.25 \ kg$
$(iv)\ 84.8 \ cm + 48.6 \ cm = 133 \ cm$

The radius of the Earth is $6371 \ km$ and the radius of its orbit around the Sun is $149 \times 10^6 \ km$. The order of magnitude of the diameter of the orbit is greater than that of the Earth by