What form factor, connector needed to replace SSD in 2015 Macbook Air?
I recently went through something similar myself. Apple uses a proprietary 28 pin connecter which means that industry standard M.2 will not work directly unless used with an adapter. I have a Macbook Pro Mid 2015 and chose to go with the OWC Aura Pro X2. You essentially have 3 options:

Get an SSD from OWC. The Aura Pro X2 provides double the speed of your current SSD. You can find a compatible SSD here.
Buy any standard M.2 SSD from Amazon and get a Sintech M.2 to Apple 28 Pin SSD adapter (Make sure you buy one appropriate to your year). There have been reports of this method causing issues with wake and sleep, battery drain, etc.
Transcend used to make SSDs for Macbooks called JetDrives. Look it up on their website.

Personally speaking, the OWC Aura Pro X2 might be your best option. The only drive that will fit in your Macbook are ones that conform to Apple's proprietary 28 Pin Connectors. This too has changes with every year. So make sure you get the year right while getting the SSD.