Where is the SSD hard drive located in a MacBook Air 2019?

It's underneath this thermal pad,

... and it's probably toast.

They are soldered directly to the logic board as others already explained. Removing it is not completely impossible, but it's certainly impracticable, and not worth the effort.

Tell your clients: if they do not back up their data, then they do not care about losing it.

It is integrated with the logic board and can not be removed or changed.

sunnywood wrote:

So if the client has caused liquid spill damage to the logic board, their data goes with it? It's pretty bad:

Hopefully they backed up to iCloud/external hard drive.

Unfortunately, likely yes.

If the T2 Security Chip inside the Mac is damaged, the data is gone for sure. That chip controls access to all of the data encryption keys.

The client should have kept data backups. This is a concept long known by computer users. Apple makes it easy to create and maintain backups: Back up your Mac with Time Machine - Apple Support

in a word yes. It's attached to the logic board.