is this a BBL jacket?

All over tiktok it says it yes just like the lulu bbl one .

I am 5’8, 36 bust, 145 lbs. usually wear a medium and prefer tight fitted. M or L for size? Thank you.

I would suggest Medium if that's your usual size. I'm 38D and 170lbs, a true size large and so I got a large. Fits tight to my body but I like the fit of it also.

Specifically looking for a nylon/spandex blend. Do all the colors come in nylon? description sates fabric varies on colors- confusing

I have this in white and the tag says the body is nylon and spandex and the trim is polyester. Not sure what they mean by trim unless it is the stitching and/or zippers. I wear a size XS or S in most clothes and bought this in size S. It fits snugly and I thought it was too small at first but it has a lot of stretch and is comfortable. I purchased this about a year ago and wear it multiple times per week and it still looks great. White is great for walking in the evening because it is easy for others to see you.

Can anyone describe the diff between scarlet dusk and chocolate burgundy?

I ordered the scarlet dusk, comparing the two jacket colors, the chocolate burgundy looks a shade darker. The scarlet dusk seems to have a more subdued rose-ish tone. There both pretty color choices. Either or, their an excellent choice.