If you need to create records in another table, you can use miniExtensions Form to add/edit records AND their linked records. The way to fill out one form that populates separate items across multiple tables is by enabling this specific feature in linked records fields.
To make creating and editing records possible, you need to enable these toggles under the "Creating and Editing Records" section. These options can be found in the linked records field setting.
Select a linked records field in your form.
Turn ON one of these options (or both).
Turning these options ON will allow users to create (or edit) linked records within the same form.
Here's an example where editing and creating records are enabled. To edit a record, the user needs to click on it for the sub-form to appear. There is also a button available to create new records.