The 38mm straps fit 40mm watches. The Apple Watch Series 4 comes in new sizes (40mm and 44mm instead of 38mm and 42mm), the good news for Apple users is that the old watch bands fit the new sizes. The bands for 38 mm and 40 mm covers are compatible with each other, and the bands for 42 mm and 44 mm covers are compatible with each other. Apple Watch Series 4, Series 5, and Series 6 come in 40mm and 44mm case sizes, so they're the same.
Recall that only four versions of the Apple Watch have the current dimensions, Series 4, Series 5, Series 6 and SE. Getting a larger Apple Watch band basically ensures that it will work with previous wearables that were the larger size option of their respective generation. I think Apple may be exercising great perfectionism here, and for most people the difference is too small to worry about. But many of them no longer include options for the 38, 40, 42 and 44 mm size options for all straps.
The watch comes in a new case size of 41 mm and 45 mm, moving away from the 40 mm and 44 mm size that has been the standard of the last generations of Apple Watch. However, the company made it a little more confusing to quickly find the accessory made for your watch size. With the Apple Watch coming in so many different case sizes, it's no wonder people get confused as to which of the different watch bands will fit the watch they have. This is because when Apple slightly increased the size of the Apple Watch case, they kept the same strap sizes.
To reiterate, bands made for the smaller of the two new sizes work with previous smaller Apple Watch models, and vice versa. So, that means Apple has to make new straps that are compatible with at least the SE or make straps that will only work for this year's models, once again unlikely to the point of being unthinkable. Apple Watch Series 7 will be announced soon, almost certainly on Tuesday, September 14 (here's why). Solo Loop and Braided Solo Loop bands are only compatible with Apple Watch SE and Apple Watch Series 4 or newer.
Since the older Series 3 bands fit all Series 6 and SE, Apple Watch fans have plenty of straps. While Apple Watch has been around for many years, Apple has maintained the same ear design every year.