Our company has been refactoring flows lately due to Apex timouts, and we're trying to decide which method runs more efficiently.

Previously, we would get a Get element to assign values of a parent record to variables; however, it appears we can also create variables on their own and then define the value of a parent record's fields as the Default Value.

Ex. {!$Record.Parent_Record__r.Value}

My question is, on the backend, is it performing a DML for each of those variables if I set the value using the example above? Which method is more efficient, and which is best practice?

Is the following case possible to be done by using SOQL.

Custom object have more than 1 feature ( like attributes), 
the relationship is like below.

Custom 1 .. N  feature

We want to recommend product to custom according to the feature.
Generally , 1 product related to multiple feature , like below

Product 1 .. N feature

I want to search the products for one custom, can I use one SOQL to do this search?