Png export

Based on my test I believe the color depth is in the millions. Here's an original color test chart in jpeg format which is 8 bits per channel:

Here is the same chart exported as a png file:

Here is the same image at 256 colors (converted in Photoshop):

It's pretty obvious that the Photos Library exported image is in the millions of colors range.

Any image I export from Photos as a PNG will show in Graphic Converter 10 as a true color PNG with more than 16 Million colors - RGB, 24 Bit, 8 bit padding, if the Alpha channel is missing.

The file size is much larger than a high resolution JPEG would be.

For example:

The PNG files are true color images, lossless, like a TIFF with a depth of 8 bit.

there is no such specification in the Photos help file - Export photos, videos, and slideshows from Photos on Mac - Apple Support

PNG is another lossless file format popular for web images.

nor in the article on PNG on Wikipedia - https://en.wikipedia.org/wiki/Portable_Network_Graphics

A PNG image with an 8-bit transparency channel, overlaid onto a checkered background, typically used in graphics software to indicate transparency.
Filename extension	.png
Internet media type	image/png
Type code	PNGf PNG
Uniform Type Identifier (UTI)	public.png
Magic number	89 50 4e 47 0d 0a 1a 0a
Developed by	PNG Development Group (donated to W3C)
Initial release	1 October 1996; 21 years ago
Type of format	Lossless bitmap image format
Extended to	APNG, JNG and MNG
Standard	ISO/IEC 15948,[1] IETF RFC 2083
Open format?	Yes

Portable Network Graphics (PNG, pronounced /ˌpiːɛnˈdʒiː/^[2] PEE-en-JEE or /pɪŋ/^[3][4] PING) is a raster graphics file format that supports lossless data compression. PNG was created as an improved, non-patented replacement for Graphics Interchange Format (GIF), and is the most widely used lossless image compression format on the Internet.^{[citation needed]}

PNG supports palette-based images (with palettes of 24-bit RGB or 32-bit RGBAcolors), grayscale images (with or without alpha channel for transparency), and full-color non-palette-based RGB/RGBA images (with or without alpha channel). PNG was designed for transferring images on the Internet, not for professional-quality print graphics, and therefore does not support non-RGB color spaces such as CMYK. A PNG file contains a single image in an extensible structure of "chunks", encoding the basic pixels and other information such as textual comments and integrity checks documented in RFC 2083.^[5]

PNG files nearly always use file extension

PNG

or

png

and are assigned MIMEmedia type

image/png

.^[6] PNG was published as informational RFC 2083 in March 1997 and as an ISO/IEC standard in 2004.^[1]

Since it is lossless I would assume that it contains all teh information in teh original photo what ever that is

But in any case I do not see any Apple statement reguarding color depth

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