Cannot access Photos app from inside programs such as Safari, iMovie, etc.

As Luis Sequeira1 says, in Safari's upload dialog, you go to the very bottom of the sidebar and select Photos.

Like many of us, you may be disappointed by the result-- you don't get albums or folders, but rather you get the entire Library of pictures in, I think, the order they were added to the Library. So, if you use this Media Selector, you may need to think ahead about what pictures you want to grab. There is a search field, so you can find pictures from June 30 2025, or you can find all the Horse pictures, and stuff like that, but you may find it frustrating.

You might want to find the pictures you intend to upload in Photos ahead of time and add keywords to make the search easier. Or, as I often do, you can find the pictures in Photos and export them to a folder in Finder. That way, you can specify the format, size, quality, included metadata, and filename of the pictures you're going to send off. Then you can avoid the medial selector and send Safari to the folder in Finder.

Just sayin'…

You do not navigate the folder hierarchy to access photos from the Photos library.

Instead, click on "Photos" near the bottom of the Open window Sidebar

(under "Media"):

MattL2 wrote: … I'm coming from a PC

Well, I'm really impressed that you thought to check the System Library setting early on. It sounds like you're figuring things out pretty well. Switching is a little like moving to England and driving on the left-- it seems pretty normal as long as you're going straight ahead, but when you suddenly need to turn, your brain turns to mush.