Hi, 

If you already _inherit other model into your custom module you can later invoke the method (function) within that inherited model.

for example if you do _inherit = ['sale.order'] you can invoke a method (function) inside that sale.order, for example an action_confirm() method which is defined inside the sale.order itself. 

Regarding the self.env['some.other.model'], this mostly used to record manipulation such as create/write/unlink records within it, and it can be directly used to call other method inside that model too. For example, doing self.env['sale.order'].search([('id', '=', 1)]).action_confirm() will allows you to search sale.order with ID of 1 and confirm it. This should work because you are inheriting sale.order into your module.

However, if you try to call action_post() that defined in other module, for example in account.move then it can be something like self.env['account.move'].search([('id', '=', 1)]).action_post(). This ORM will search an account move record with ID 1, then later validate it.

In the question you asked, you should add action like search or create or unlink into self.env['some.other.model'] before invoking foo_fii() into the ORM, depends on what foo_fii() method does to the records :)

I hope this helpful, if it does kindly please mark the answer as solved for future reference :)