Reflections on SI units, Uncertainty & s.f.

In S.I. units, I learned that the derived units used in chemistry is the product or quotient of the based units. And prefixes are used to ensure that the result has less number of zeros. For uncertainty, I learned about the two rules, one is the rules for addition and subtraction, the other is rules for multiplication and division. Lastly, I need to use the 5 rules to determine the number of significant figures inn a number. Firstly, all non-zero digits are signficant, secondly, any zeros between two digits are significant, thirdly, any zeros to the right of a decimal point till the end of the number are significant, forthly, any zeros to the left of nonzero digit are not significant. Lastly, the trailing zeros in a number without a decimal point may or may not be significant.