Actually, Odoo automatically apply sequence on ID field. This is done by _sequence attribute on model and it's apply on 'ID' field here https://github.com/odoo/odoo/blob/12.0/odoo/models.py#L3630

If you check your table in Postgresql then ID column have description like id integer NOT NULL DEFAULT nextval('model_name_id_seq'::regclass). Here, nextval will execute sequence which will auto increment index.

You can read in detail what is exact nextval function? https://www.postgresql.org/docs/current/sql-createsequence.html

You can use _sequence attribute on model to change sequence field.