Since MongoDB schema-less so when you create a schema Aventum convert it to a schema understandable by MongoDB, however, if you used SQL based database like PostgreSQL or MySQL then here is a close description of what Aventum do to achieve the dynamic schema.
On creating a schema Aventum creates a table with the name same as the name that you put when you created the schema, with the fields that you added however if the field is repeatable and the repeatable field is Relation or Upload then Aventum considers this as many-to-many relationship and will create a third table with the name ${schemaName}-${reference}-${fieldName} with columns ${reference}Id(if the content has relation with itself like for example categories have child categories then this column will be child${Reference}Id) and ${schemaName}Id the reference here is the uploads table or the relation table.
If the repeatable field is a select field then Aventum will create a table for this field with the name ${schemaName}-${fieldName}-options and save the select field options within it, then will create another table with the name ${schemaName}-${fieldName} with ${schemaName}-${fieldName}-optionsId and ${schemaName}Id columns.
If the repeatable field is not a relation or select or upload then Aventum will create ${schemaName}-${fieldName} table with ${schemaName}Id and value columns and the value columns will have the same field type.
The "custom" field will be saved as a JSON string in a text column type no matter if it was repealable or not.
To see how Aventum creates these tables take a look at AventumFolder/server/models/sql/schema.js
To see how Aventum saves the content data within these tables take a look at AventumFolder/server/models/sql/content.js.
Aventum saves the schema settings in the schemas table/collection.