DELETE key on Macbook Air 2019 -- after being cleaned -- cannot be snapped back into place.

If none of the nubs on the butterfly mechanism are damaged, then inspect the two plastic clips on the underside of the key cap. These will typically break off or become bent preventing the key cap from locking onto the butterfly mechanism. You can problem find replacement key caps online. Except for language & regional differences, I believe the key caps are all the same for them. Just make sure the replacement key cap is for your exact model.

Technically Apple or an Apple Authorized Service Provider should have access to key cap replacement kits, but from reading this forum I have yet to see any users post that an AASP replaced an individual key cap. Since Apple only provides full kits to an AASP, you would likely need to pay for the full kit containing each of the other key caps for every key on the keyboard.

FYI, I know that Apple changed the font & styling for some keys so a replacement key cap may have a slightly different look than the original. Sometimes I have seen differences in the color even though they are all black keys.

Unfortunately the keyboard offers no serviceable user parts. You'll need to bring it to an authorized service center to help you out. In the meantime, getting an external USB keyboard might be helpful.