Fisker outsourced everything.
Fisker who?
Valmet Automotive build the A Class Mercedes, they used to build the Porsche mid-engine products and plenty of other quality cars. It went badly wrong with Fisker.
Outsourcing is no magic bullet, design, management, sales, marketing, supply chain management, finance and plenty of other stuff has to happen.
The bit we only ever see on the news is final assembly - the production line. The sub-assemblies are not shown.
There was a time when things like seats were made in house by the auto makers. Over time the likes of Johnson Controls have ended up doing all of that. The 'persons needed to make a car' goes down dramatically if you have a guy and a forklift dropping off the seats at final assembly with those seats coming from a supplier. If you have a gigafactory sized floor full of women and sewing machines making seats, dashboards and steering wheel covers then 'persons needed to make a car' goes up considerably.
There was a famous strike in the UK where the women that made the seats went on strike - allegedly they were not getting equal pay. Ford could not sell cars without seats in them so it was not a good situation.
The real reason for outsourcing some parts and not others is because the outsourced components can be better than what you can make in house, even if you are Elon Musk.For instance, in a crash you do not want that plastic dashboard to shatter into pieces that can kill the driver, you want it to disintegrate more gracefully. Although the Tesla may be with its own funky design the interior is almost certainly mostly outsourced to the same companies that make all the other interiors but specified differently.
The example in the Munro teardown of the rear view mirror, that is something that can be specified correctly for a contractor to make, for someone on final assembly to just glue in place. The other brands - GM and BMW presumably have traditional embedded electronics driving the thing, not some high end 'PC' tucked under the back seats. So they have to specify a more convoluted mirror.
Tesla did learn from the original Roadster on what they needed to do to build a car. There was a poor cultural fit between what they were trying to do and their 'leaded petrol' friends at Lotus etc. were doing. As evidenced by the unfortunate Fisker Karma there is no way to magically build a car just by getting the whole thing outsourced.