How to make hamburger buns as fluffy as in McDonald's?
The light commercial white bread structure is very different from homemade doughs. I'm learning how to achieve it, so here are some pointers:

The main objective is to make your gluten work really hard: it has to hold up twice as much air as your regular homemade bread.

Autolyse your flour first, just mix it with the water and let sit for 20 minutes.

Don't add any fats before hydrating the flour and kneading it to activate the gluten. The fats are very helpul afterward, they coat the gluten strands and keep the finished bread soft for a longer time. If you add the fat first, it keeps the gluten from activating properly.

As has been stated before, knead the dough intensely and for a long time, i.e. 20 minutes in a stand mixer. The commercial kneading process is very intense and you will see the difference in the dough's texture and ability to hold a higher water content.

Adding more wheat gluten is also a good idea.

Do tangzhong a.k.a. water roux. Take a bit of flour (recipes vary from 3 tablespoons to 1/3 of all your flour), mix it with water and heat it in a water bath pot to a temperature of 60-65 Celsius while constantly whisking to keep it from clumping. It's going to achieve a gel consistency. At this temperature, amylase enzymes activate and change some of the the starches in your flour to sugars. For best results you should hold the mixture in this range for some time, at least 10 minutes. The same process happens in your oven while heating the loaf at the start of baking. Baking at a lower temperature prolongs this stage, but once the dough reaches temperature of 68 C, amylases are deactivated for good. Some commercial bakers also just add amylase or diastatic flour for this purpose. 

The commercial-type bread therefore has less starch, more sugar and more protein (gluten).

There are also other dough enhancers that make mass produced bread soft, fluffy and long lasting. Vitamin C (used in minimal amounts) makes the proofing faster and helps keep the product soft for longer. Emulsifiers like lecithin or xanthan gum can be used to hold up the gluten structure and also keep the bread fresh.