To download multiple photos, you have to export them from your iOS PhotoRoom application.
To do so:
Follow the tutorial on How to export multiple photos at once
At the export screen, select the first icon named "Desktop / Export to Web"
Once it's exported, go to https://www.photoroom.com/login/ while logged in
You can now view the latest creation with the number of images in it. Like this 👇
And now you can download it by clicking on the image itself. Wait for all images to be downloaded before closing the window.