I don't work at Apple but I am a mechanical engineer at a tech company so I can take a stab at it.
There's two paths I see for a mechanical engineer at Apple, and for most tech or consumer device companies.
First is the product design, which does involve a lot of shuffling around components in reaction to EMI tests but there's a lot more. If by optimizing manufacturing you mean improving yields and lead times, yea there's definitely some of that. But also a lot of the mechanical design for Apple components usually requires new tooling designs as well and really pushes the limits for precision machining. Also you could be doing research into new materials if the readily available stuff isnt up to snuff for your super thin designs. There's also a lot of emphasis on UX, and I don't mean fancy digital borders and and fonts. How does the device feel when you hold or touch it? How easy is it to open up your macbook, or hold an iPhone?
If you want to know more, Apple does release some videos that go into detail about the mechanical design of their components. Here are a few I found:
Then there's the reliability side of things. It's much less product design and more beating the crap out of devices in the lab. You might have to design test fixtures to make sure the tests reflect whats happening in real life, do a lot of number crunching to say what's an acceptable failure rate for this device, or really dive deep into material science to see why things are breaking.
Mechanical engineering at Apple is a lot more than just shuffling components around. You work on a much smaller scale and a lot of stuff you would do is hidden and most people don't think about it. However getting the mechanical design just right makes for a much more pleasant user experience. However, if you're not interested in this kind of stuff, there isn't much for you at Apple. You'd probably like Tesla, Google [x], Space X, or any aerospace company better.