There are plenty of design engineers on here who can offer you explanations as to the complexities of u/c design, but this may help in the interim
https://aviation.stackexchange.com/q...g-gear-tilt-up
As for the 737, the outer wheels on the main u/c have a hub cap. When the gear is retracted, around the edge of the wheel well are some very effective and robust flexible rubber seals which act as wheel bay seal. There are also fixed fairing doors attached to the outside of the leg to complete the seal.
You really have to see a 737 jacked up, with the gear retracted, to understand how this simple, but effective, design works. There's no discernible noise for the pax in the cabin either.
If there are performance related issues, again, that's one for the designers to explain.