To edit multiple images at once, you need to create a batch. You can then use the Batch Editor to make the changes.
Steps (Android)
To create a batch of images:
Select the Space you want to work in.
Select Batch.
Select Start new batch.
Alternatively, select Resume previous batch to continue with your last batch. Photoroom opens it in the Batch Editor, and you can skip the remaining steps.
Select the images you want to include in your batch and select the checkmark icon to confirm.
Photoroom creates your batch and each image keeps its original background.
You have now created a batch. The images appear in a grid in the Batch Editor and you can use the Batch Editor features to make changes. For details, see Edit multiple images at once in Batch - workflow (Android).