Gutenburg is a fundamental change to the way you create content. Instead of writing paragraphs and headings and adding media, you will now use blocks. If you are already using Gutenburg you will know how to use blocks. If you aren’t using Gutenburg then you can ignore this page until you upgrade to WordPress 5.
The Loan Calculator block is in the Widget section:
Once you have selected the Loan Calculator block you can set which form you want to use:
If you want to go back to building your pages the old way you need the Classic Editor plugin.