Finding specific things
Your photos are files on your disk. Searching for one by name finds nothing, because the library is a package and macOS treats it as a single object.
~/Pictures/Photos Library.photoslibrary/originals/Inside, originals are filed into subfolders named 0 through F — the first character of each file's UUID. Filenames are UUIDs, not the names your camera used.
That is the second surprise: even browsing the package, IMG_4021.HEIC may not exist under that name any more.
Right-click the library in Finder → Show Package Contents. Read anything. Change nothing — Photos maintains a database that must agree with the files, and editing them by hand corrupts the library.
Select photos in Photos, then File → Export:
For a whole library, File → Export → Export Unmodified Original on a large selection works, though it takes a while.
Two reasons stack up. The library is a package, so Spotlight indexes the container, not the contents. And the filenames inside are UUIDs, so even a search that reached them would not match what you typed.
Photos' own search is the intended tool, and it is genuinely good — it searches dates, places, faces and detected objects rather than filenames.
Photos can be pointed at a folder-based workflow by turning off Copy items to the Photos library in Settings → General, so imports reference files where they already are. That is the setup to choose if being able to find images as ordinary files matters more to you than having one managed library.
Safe to look. Not safe to move, rename or delete anything — the library database will no longer match and Photos may refuse to open.
Photos renames imports to UUIDs so two files with the same camera name can coexist.
Everywhere keeps track of every file on your Mac — including the folders Spotlight hides and drives you have unplugged. Free for a day, then $19 once.
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