Can I connect an SSD to an A16 iPad's USB slot?

Yes, with caveats...

To be recognised by iPadOS, the attached USB/SSD storage device must be formatted to FAT, FAT32, exFAT, APFS or HFS+ with a single data partition.

If you have one of the newest iPad Pro models, with a USB-C connector, you may need to utilise a powered USB hub (i.e., it has its own source of power) - connecting both the iPad Pro and the storage device to ports on the hub.

Beware of SSD write-caching. iPadOS does not have a “safe” or “Quick Removal” option with which to safely dismount a mounted storage volume. The only way to safely disconnect an SSD volume from iPadOS is to fully shut-down the iPad prior to disconnecting the SSD - as this is the only way to ensure that any pending “write” operations are competed before disconnect. Failure to observe this precaution can result in file corruption.

You can find information about connecting USB storage to your iPad here:

Connect external devices or servers with Files on iPhone - Apple Support

https://www.imore.com/how-use-external-hard-drives-your-iphone-or-ipad

Yes, you can connect an external solid-state drive (SSD) to the USB-C port on an iPad with an A16 chip(such as the iPad 10th generation or newer compatible models).