The only components you are able to use within an LWC are your own or those specified in the components library. It is not possible to embed components such as the "Related Record" one into your LWC.
However, what you could consider is splitting your LWC component into two or more smaller components that you place into the Lightning Record Page in a layout that makes it look like the related records list(s) are embedded within a larger component (e.g. having your component provide a "header" above and a "footer" below the related records list(s)).
An alternative is to consider writing your own related records component that looks similar to the OOTB one using the SLDS blueprints (such as Data Table).