I have done below modification in your website:
1) Edit the taxonomy view "Ad Subcategory"
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in section "Query Filter", change the filter as below:
Select taxonomy terms whose parent is the current taxonomy archive.
2) Edit the taxonomy view "Ad Sub-Subcategory"
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in section "Query Filter", change the filter as below:
Select taxonomy terms whose parent is the value set by the parent view.
You can test the result in front-end, for example:
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You will be able to see the sub terms and related posts.
There are new lots of question in this thread, we can handle them one by one.
Q1 )Is it best to use the Bootstrap Grid - 4 columns?
That depends on yourself, you can setup it as what you want, here is a document about
https://toolset.com/documentation/user-guides/view-layouts-101/
Views Output Formats – Bootstrap, Table, Grid, Ordered and Un-Ordered Lists
Q2) Can I add a white textured background image to the taxonomy display pages with Divi?
This is a DIVI question, it should be able to achieve with a little CSS codes. for example:
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Q3) Sitemap (for entire site): hidden link
I assume we are talking about the taxonomy view "Ad Cat Sitemap-No Image"
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1) it is a taxonomy view, it does not custom search form, so it is not possible to setup a search form as you mentioned above:
SEARCH at the top for TYPE Section (different Sections), Countries, States, Zip, Level 1, Level 2, Level 3 dropdowns.
2) Even in post form, it does not setup the dropdowns by taxonomy field, it only support the post type relationship, see our demo site:
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section "Search Houses", the dropdowns "State" and "City", it is based on post type relationships:
State-> City -> Property
You can setup your own demo site here and check the settings
https://discover-wp.com/site-templates/
Q4) VIEW: for Businesses at Level 4
I think in your case, you just need to customize the wordpress archive page of taxonomy "Ad Categories":
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Here is a document about:
WordPress Archive – Customizing the Appearance of Archive Pages
https://toolset.com/documentation/user-guides/normal-vs-archive-views/
Please create new thread for the new questions, that will help other users to find the answers. thanks