I have a water damaged 2015 MacBook Air which will not turn on. I would like to sell it for parts but want to firstly erase all the data from it. I know that one method of doing so safely is just to take out the drive manually, but is there some method to do it for example from another MacBook?
First, the SSD is PCIe based so it's removable. Open the back cover and remove the SSD. There's an excellent step-by-step guide on iFixit.
Removing the SSD will ensure that nobody else will have your data and at this point, you can list your MBA for sale without the fear of your data ending up in someone else's possession.
You then have the option of installing it into a USB enclosure so that you can access the data (via the other Mac), though there's no guarantee that it (SSD) will work. If it does work, you can copy your data off and then wipe the drive and sell it as well.
Could you install it into a similar MBA (mid 2013 through 2017 MBAs)? Sure, but I am hesitant to put electronic components that are of questionable functionality into working machines. I personally wouldn't take the risk.