Is the LaCie external drive sold on Apple website formatted in APFS?

Probably not, but it’s irrelevant. All you have to do is connect the disk, launch Disk Utility on your Mac, select the new drive in the sidebar on the left then click Erase and format it as APFS (GUID partition scheme). Then it will be fully compatible.

Caprese52 wrote:

Looking for disk compatible with Big Sur - does this one work?

All new drives need to be formatted.

Even drives that say "Mac ready" or some such, typically they load their own proprietary backup software.

You want to insure proper erase/format/initilize on your own terms, and get rid of any garbage—easy to do:

Erase and reformat a storage device in Disk Utility on Mac

https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/19.0/mac/10.15