You are correct in that Scott won't show most every stamp in the world; they do usually provide a description in the footnotes for a series; such as 1 cent - Bob Smith, 2 cent - Steve Jones, 3c - historical building and so forth. I've used
ebay and Hip from time to time to identify or confirm that I'm buying a correct set though if that's your issue. I just search based on catalog number and check out the listings to make sure I'm buying what my (Scott or SG or other) catalogs confirm.
If you don't mind spending the time, you can usually find out what you are looking to identify online. Some stamps will be harder to identify online due to lack of specialists or specialty websites, but that's totally dependent on what you are looking for. To answer your question though it will depend on the catalog in regard to photos/scans but in general you are not going to find catalogs that show all photos of the same design for an entire series.
If you really do want to specialize, then Sakura is the go-to for Japan, Yvert for France/Colonies. Don't be at all afraid to buy used catalogs unless you are trying to collect right up to present day. I'm still using 2015 Scott catalogs for WW and a 2018 for US when needed and they work just fine.
I will add that if you really want to dive in to any particular area of philately; own a catalog that covers what you intend to collect. Online look-ups are fine but you sometimes won't get the details you might want like perf/shade/watermark varieties that you will get from a catalog.