What cable to connect iPad to SSD drive?

As your iPad Pro 11” has a USB-C port, you’ll need a USB cable that is suitable for your SSD drive - and has a USB-C connector for your iPad. In practice, this will likely be a USB-C to USB-C Data Cable.

To be recognised by iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT, APFS or HFS+ with a single partition. Microsoft NTFS is not supported. Formatting and partitioning, if required, must be performed using a separate computer; iPad does not provide any facilities to format an external storage device.

As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source may required. 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 SSD storage device to ports on the hub.

Some additional information can be found 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

The tags appended to your initial post, selected by you upon submission, indicate that you have an iPad Pro 11” WiFi.

If you have an iPad Air4, with a USB-C port, then my original comments apply.

Should you instead have an iPad with a Lightning connector, you will require a Lightning to USB3 Camera Adapter and a USB Power Adapter capable of at least 12W output.

• Lightning to USB 3 Camera Adapter

https://store.apple.com/uk/xc/product/MK0W2ZM/A

As this adapter features a USB Type-A connector, a matching cable would be required.