I wasn't there for the 360 controller's design, but I'd assume MS hired external companies to assist with some of the industrial design work.
The xbox org in general had a very high rate of contractor/vendor usage, I saw plenty of projects (including the one I was on) that had well over 50% of employees as vendors.
The design team I worked with was also a mix of MS employees and external designers, which makes a lot of sense.
I can tell you that in regards to the Xbox 1 controller, Microsoft employees spent a lot of time showing love and care to it, those buttons and control sticks got a ton of critique from people who really care about gaming. I remember one particular test session for haptic feedback settings, everyone there wanted to make damned sure that if you were driving over gravel in Forza that it felt like you were really driving over gravel.
I can speak more to the Microsoft Band. The springs on the buttons were iterated on for months, with dozens upon dozens of combinations of springs and button materials being tested to find what felt just right, down to the exact shape of the indent on the button.
Every single letter for the font was hand kerned. Heck the font was customized for our exact display (DPI, font weights, letter heights, everything). My team worked with Monotype to get real TrueType font rendering working in 96mhz and 256KB of RAM (and a slow 4MB scratch buffer). Real antialiasing, full CJK support, when Microsoft wants to pay attention to details they can do a really damn good job of paying attention to the details.
Xbox was (is, I presume, haven't been there in a long time!) an entire technical org that was passionate from top to bottom about making the best products.
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