Presets are saved to a file in the app data directory. Choose Settings > App Data Directory > Show… and then look in the presets folder. The names of the sub-folder are internal names because the user-visible names are translatable and also the English name changes sometimes. Here is an mapping of some of them: Preset Locations: Filters.
advanced color grading
You can do secondary color correction using Mask filters as explained here: Mask: Apply
Built in library effects
I do not want to make the download much larger than it needs to be; so, this would require development of a cloud system and client UI within Shotcut to browse, search, and download them. Then, there is the issue of curating from the many royalty free sources. And if users can upload then, then a moderation and DMC takedown notification system is required. That is a lot of work that is less important than other things on our Shotcut - Road Map. So, instead we have a Shotcut - Resources page and the Resources category in the forum.
Why not like add this when the video is already selected and THEN you can manipulate it?
On the road map link above is an item called “quick effects VUI” (“VUI” means either video or visual user interface), which is similar to this request. It would be a vertical toolbar on the left or right of the player that will add (or select the first) filter for common things that typically have on-video controls such as Size Position & Rotate, Crop: Rectangle, Text: Rich, and White Balance. But I understand your request as I have seen this in other tools. Maybe