PSD
The native layered file format of Adobe Photoshop — it preserves layers, smart objects, masks and effects, and is the format most professional mockups are delivered in.
Also called: Photoshop document, PSB
PSD (Photoshop Document) stores an image as editable layers rather than a single flattened picture, which is why it is the working format for mockups: the scene, the lighting, the shadows and the replaceable smart object all live on separate layers you can adjust.
For very large files Adobe uses the related PSB (“large document”) format, which lifts PSD’s 30,000-pixel and 2 GB limits. Both open in Photoshop, and most professional tools (Affinity Photo, Photopea) read PSD with varying fidelity.