Welcome to the Bunnings Workshop community @AdeleN. It's wonderful that you've joined us, and many thanks for jumping into the discussion.
That's an excellent question, and I'm struggling to work it out myself. We do have the Whites Outdoor 180mm Black Round Deluxe Balcony Pot Holder and Whites Outdoor 200mm Black Pot Holder, which could be used for a similar purpose. There appears to be a fixing in a couple of the pots, suggesting the rim of the pot had a hole drilled in it, and it's been screwed to the timber. The only way I can think of to successfully drill into terracotta would be with a multi-material drill bit in a drill operating in non-hammer function mode. However, let me mention @JP_Finlay to see if they could fill us in on what was used.
Have you taken inspiration from this project and were wanting to replicate it? I trust our members would love to follow along, and we'd encourage you to share it with us. Please do let us know if you need any assistance.
Welcome to the Bunnings Workshop community.
Hopefully @JP_Finlay will be able to get back to you soon. You also might be interested in this project by @DIYology - Terracotta pot vertical garden screening. And do check out this new article - 5 ways to create a vertical garden.
Hope that helps.