You can open images from the Photos app to edit in Pixelmator Pro and then save the changes back to the original image in your library.
Use this method to save images while preserving all layers, adjustments, effects, styles, and other formatting.
Note: If you're seeing a flattened copy of your edited photo when you open it using the Pixelmator Pro Photos extension, make sure saving changes "For Images Opened from Photos" is turned on in Pixelmator Pro Editing settings.
Alternatively, you can turn off "For Images Opened from Photos" not to preserve edits and overwrite your original images with edits made in Pixelmator Pro. The edited images can always be reverted to the original using Image > Revert to Original in Photos.
Use this method to overwrite original images with changes, merging all layers, adjustments, effects, styles, and other formatting.
Note: When opening images using Edit With, additional data of RAW files is not kept. Images are converted to TIFF file format instead.
If you've created a project on your Mac and would like to move it to Photos together will all nondestructive edits and layers, see article Use the Pixelmator Pro Photos extension to learn how to import your Pixelmator Pro documents to Photos.