This page explains the difference between single and double quotes

If you use single quotes, it will be the literal string. For example, the variable is $exa, which is supposed to equal 1. But since it is in single quotes, it will stay as $exa

However, if you use double quotes, the variable $exa will be translated: 1.

Using the same line from the single quote example but now in double will turn into this:
If you use single quotes, it will be the literal string. For example, the variable is 1, which is supposed to equal 1. But since it is in single quotes, it will stay as 1