Every AI-generated background is created specifically for each individual design. This is because the lighting, textures, and other factors are unique to each image, and so have an impact on the background. For this reason, you can't use the same AI Background on multiple designs. But you can create similar backgrounds that look like part of a themed collection.
To create similar AI Backgrounds:
Use the same text prompt or image prompt for each image
Choose a similar-looking suggested AI Background.
For instructions, see Add AI Backgrounds and Create your own AI Backgrounds.
On the Web App version of Photoroom, you can use AI Backgrounds and Batch Mode together. This lets you set AI Backgrounds for a collection of images, all at once. For details, see Batch edit with AI Backgrounds and AI Shadows.