InnoDB is similar to SQLite all tables are clustered indexes, if you don't have suitable primary key it creates a hidden one similar to rowid in SQLite.

MSSQL and Oracle give you a choice for table storage to either have unordered heaps or ordered b-trees, depending on whether a clustered index is defined.

PG only has unordered heaps, indexes are always secondary data structures. PG could really use true clustered indexes as an option.