By default SXA comes with a computed field
sxacontent which is aggregating all texts from all fields of an item and uses the implementation in
Sitecore.XA.Foundation.Search.ComputedFields.AggregatedContent. It is configured in
\App_Config\Include\z.Foundation.Overrides\Sitecore.XA.Foundation.Search.Solr.config. Thanks to this there is no need to configure fields and other stuff, you just drop Search Box and Search Results on the page and search is working.
If I understand correctly from your question you would like to index content from the items selected in the droplink, tree list fields? You can add your own custom implementation if you override the configuration node
<field fieldName="sxacontent" returnType="textCollection" type="Sitecore.XA.Foundation.Search.ComputedFields.AggregatedContent, Sitecore.XA.Foundation.Search">