The tool looks very cool! But IMO you can't get an ER diagram from SQL since entities are fundamentally different from tables. They are certainly very similar, but SQL alone doesn't give you enough information to create an ER diagram. That's not to say that the tool is useless or that diagrams of this sort are unhelpful. I'll admit I'm being pedantic and others will probably disagree.
Can you please elaborate on the differences? They are practically interchangeable, but conceptually there might be another layer on top of entities and relationships for somewhat richer semantics (like describing a relation, or additional annotations on the entity)
Many to many will lead to an extra table (which can have additional properties, requiring this table to be modelled as an entity), one to many leads to the inclusion of a foreign key to another unique key (referring all columns of that key, _id is an implementation decision, compound primary keys are possible). One to one can be implemented in multiple ways, like one to many with a uniqueness constraint by the referring table or even by merging entities to a single table.
The raw SQL can be revealing but when entities have merged into one table it’s harder to tell what is what, unless a certain set of columns appears over different tables.