This is a common question, especially when customers upgrade from EE and they have saved queries that should be converted to advanced queries. It seems there is no information available for IFS Cloud advanced queries though this has been a topic from 21R1.  

Using basic search first, takes one only a bit forward. Would be interesting to know how nested queries work with advanced search, or what query syntax one should use with advanced query editor. Is it odata?

How to find customer orders based on customer country would be a good example of using advanced query, as the country attribute is not available in the Customer Order view/projection. 

As I know this is OData filtering.

Start with this:

https://learn.microsoft.com/en-us/azure/search/search-query-odata-filter

I'm looking for some user-friendly documentation, or even consultants who are less technical. But thanks for the link.