Question: Are template parts blocks?
Answer: Yes. Template parts are blocks. They contain inner blocks which can include other template parts.
Example template part
<!-- wp:sb/breadcrumbs-block /-->
<!-- wp:heading {"level":3} -->
<h3>Navigation</h3>
<!-- /wp:heading -->
<!-- wp:sb/sb-prevnext-block /-->
<!-- wp:oik-sb/children /-->
<!-- wp:heading {"level":3} -->
<h3>Page List</h3>
<!-- /wp:heading -->
<!-- wp:page-list /-->
