In Odoo 14 I can search a external id of a record with:

self.env.ref('simulator.example')

But I need to get the ID of a model _named simulator.simulator and this code:

self.env.ref('model_simulator_simulator')

returns:

 File "/usr/lib/python3/dist-packages/odoo/addons/base/models/ir_model.py", line 1873, in xmlid_lookup
    module, name = xmlid.split('.', 1)

ValueError: not enough values to unpack (expected 2, got 1)

In theory, am External ID is just a record with a name and a number that reference the real id in the database table. It doesn't seems a problem with the name, because the error for wrong names is 'Externa ID not found...'.

Why it fails searching models, actions and fields and works with records?

Thanks