Just for the record, the related model is res.company.
You can not simply change a Many2one field to a Many2many. While I usually tend to not say that a task/change is impossible to perform in Odoo, I'm rather certain you've hit one of the few dead ends, especially when trying to do this in Studio - but also when doing this on source level, unless you're up for heavy customization, even heavier testing and still bug-fixing as you go.
The thing is, the way, a Many2one fields is written information to and read from, is fundamentally different from a Many2many. And since we speak Inventory, or ERP on a broader scale, "Company" and "Product" are, as you can imagine, deeply nested concepts, affecting tons of other applications/functionality within. One main concern in changing that field further down the line would be access/security rules that are based on a single company (or none) type of field globally in Odoo.
So, generally speaking, you can either have a product be relevant to just one company (by setting exactly that res.company Many2one, or have it available to all companies, by not setting that field.
Side note: When using AI, it's always important to define a strict scope and context detailing as much as possible to receive relevant and useful information. Odoo AI is no exception to that (at least for the moment).
