Personally, in another life, I'd like to be a lens designer at Nikon or Canon. I think it would be so cool to put a lens from scratch together and test it out. Not only that, but it would be my job to know lenses of the company inside an out so I'd be able to use every lens in the lineup as well, on every camera!
Way back in film days In Australia amateur me somehow got into the pro group dealing with the then private Nikon importer and I could borrow any lens I liked to play with for a few days or longer on my Nikon N8008s. That was an interesting time. The borrowed f/2.8 zooms were giving me nice results but long term I could not handle the weight and bulk. Luckily in this digital life I went M4/3 and the f/2.8 zooms there are a totally different carry-able difference.
Once was asked to do a mini casual review for the importer on a a relatively cheap Nikon tele zoom lens and it turned out to be way better than expected if used correctly, so bought one for myself.
I certainly can't afford to try out every piece of equipment but I would do so if I were a lens designer!
I would suspect lens designers would be chained to their desk and the computer and not allowed outside to play.
To sample stuff, become a sales rep for the company in question. Or get filthy rich and buy everything - sadly I only managed to get filthy.