Having wanted to buy a DSLR since long, I chose a brand first. Nikon.
Went with the most affordable (to my pocket) camera D3100 which came with a basic 18-55 lens kit.

This was 2 years back. I did not know anything about pro-bodies or lenses at the time.

Gradually, as I started investing more time into photography, I realized that lens lineups not only vary across different platforms, they vary on their own platform as well.

Last year, I wanted to upgrade my lens to a 1.8 aperture lens and found the 50mm 1.8D to be in the affordable range. However, this lens does not auto-focus on the entry-level Nikon camera bodies. It would fit, but with only manual focusing. The 50mm 1.8 AF-S (autofocus) lens which would work for my camera was more than twice the price.

This is because Nikon entry-level bodies do not offer an "in-camera auto focus motor" required to auto-focus older lenses. This feature is only available in mid-upper-to-pro-level-camera-bodies.

Manual focus is fine for object or table-top photography, but I myself did not find it useful elsewhere for my work.

Also, the 50mm lens was also better suited for a full-frame body and on my camera the effective focal range after mounting a 50mm would be 75mm due to the added 1.5x APS-C crop factor. Sounded Ok for portraits or tight-shots, but not for general everyday photography to me.

I ended up purchasing the 35mm 1.8 lens instead which auto-focuses on my camera and would give a 50mm equivalent focal range.

Comparatively, Canon does not have mount/auto-focus issues with their currently available entry-level or mid-level cameras and lenses. Canon also does not offer a "budget-range" 35mm lens (50mm equivalent) for the crop-sensor body. The current available Canon 50mm 1.8 lens would work, but with a focal length of 80mm due to the 1.6x APS-C crop. Other manufacturers or micro 4/3 cameras would have a 25mm lens with the body to achieve a similar focal range.

I also have a group of friends with Nikon cameras and share multiple different lenses from time to time.

Also, with entry-level-mid-level cameras, having luckily found the time and chance to use Nikon, Sony and Canon since past two years, the kit lenses 18-55, 18-135, 55-200, 55-250, etc. on all three platforms perform quite well based on the shooting technique.

I have also used the Nikon 105mm 2.8 macro and the 300mm F/4 on the Nikon D3100 camera body and the results are most exquisite just because of the lens quality. Similar lenses on other platforms would also perform equally well but in the end, you need to figure out your style of photography first.

To answer your first question, yes, there is a difference between platforms, but final image quality depends on your shooting style. Like, in printed images (such as magazines) or those which you see on the internet, you will have a hard time figuring out the manufacturer + lens make just by looking at a photograph. Wide/tele can be figured out - but how far was the final image cropped before publishing is still a question.

For the second question, you "can" pick the lens first and then the body but that would only be the case in which you have first figured out your photography style - portraits, wildlife, macro, sports, etc. If you have more than 2 different styles - like for example, you are a wedding as well as a wildlife photographer, then you need to choose a platform which provides a more extensive lens choice.