What is the difference between PSB (Photoshop Big) and PSD (Photoshop Document)?
If you check the Adobe help website it's stated right there:
Photoshop format (PSD) is the default file format and the only format, besides the Large Document Format (PSB), that supports all Photoshop features.
The Large Document Format (PSB) supports documents up to 300,000 pixels in any dimension. With documents larger than 30,000 pixels in width or height, some plug-in filters are unavailable.
Like most file formats, PSD supports files up to 2 GB in size. For files larger than 2 GB, save in Large Document Format (PSB), Photoshop Raw (flattened image only), TIFF (up to 4 GB), or DICOM format.
the PSB file remove the limit of 30 000 pixel image size so when you work on a big size print project you gonna use psb instead of psd file :)
If you want very detailed information you can check PSD/PSB file specification: https://www.adobe.com/devnet-apps/photoshop/fileformatashtml/
Look for PSB tag in that document.
PSD has 4 bytes for bitmap dimension size but PSB has 8 bytes.