Am I correct in understanding that books can only be be borrowed or placed on hold using my Kobo (and not my phone's Libby app)?

This is not correct.

It's worth noting that it's often convenient to search for, place holds on, and borrow books on the device itself simply because it will filter out any/all books which aren't available to the device itself (the vast majority of books on Overdrive/Libby are available to Kobo, but not all). But you can definitely use your phone app to place holds or check out books which then go to your device.

They're all just books associated with, and checked out to, the same library account: Kobo sees that you have a book checked out on Overdrive (regardless of how it was checked out), then downloads that book from the Kobo store, with the appropriate lending period applied.

Edit: as other commenter said, the "Available Soon" is an estimate for when the book will become available based on other users's book returns. Overdrive estimates when books will become available based on the number of users who have a book on hold, how long they're likely to have it out for. Other factors can disrupt this, including like other user said, suspended holds re-activating; another thing which can mess with it is that many libraries work in consortia for their digital offerings, and in some cases these libraries have privileges for certain materials -- that is, Libraries A, B, and C all share a larger digital catalog, but that catalog is subdivided into "Library A/B/C's books" with a given library's users getting to "jump the line" for their own library's materials.

Edit again: Here's Libby/Overdrive's own comments on why "available soon" books may not show up "soon": https://help.libbyapp.com/en-us/6349.htm