The biggest downside of the most recent line of MacBooks is that they are unable to be upgraded. The memory and storage chips are all soldered directly to the logic board.
However at least with the MacBooks (both Pro and Air) made 2012–2016, there is a workaround. You can use the SD card slot to add more space. If you use a standard SD card, it will stick out from the edge. But you can get a micro SD card and a short SD card adapter, which will sit flush with the case (or at least close enough for practical purposes).
It's my photos, videos and music files that are eating up all my space. So I bought this 200GB micro SD card to use as and expansion to store my media files. (I looked into the 256GB ones, but they were nearly double the price for only 28% more storage.)
There are several different short SD card adapters out there. I tried the one by Adafruit designed for the Raspberry Pi, but unfortunately it doesn't work on the Mac. Or at least it is intermittent so I can't count on it. I ended up going with a BaseQi adapter. It's more expensive, but the review are good and it is designed specifically for this purpose. Be sure to get the right one! There are several different sizes, each designed for a particular year and model of MacBook. There are also some by 8mobility that look reasonable too.