why searches for file names in photos does not find images

Try searching for "123456.jpg" specifically in a smart album.

With Photos 5 or 6 original files have been renamed to a nonsense (to us) file name to make it easier to sync with iCloud Library. The local library retains the file name in its database and will use it what sharing or exporting out of the library.

If you want to get the file name with or without the extension into the title field you can use one of these AppleScripts:

Script: Batch Changing the Titles to the Filenames with Extension

Script: Changing the Title to the Filename without Extension

The scripts compiled as an applications can be downloaded from this tutorial site:  P01 - Applescripts from Photos’ User Tips Compiled as Applications  

The scripts don't work in Smart Albums or in nexted folder/albums. Also the first time running the app Control (right) - click on it and select Open from the contextual menu.

Unfortunately, no way I've found. I can find parts of a name only of it is just after an underscore:

Eg. IMG_123456

If I search 123 it will display. If I search 345, nothing. *345 doesn't work either. I tried making a smart album with title contains 345 and same kind of results. Images beginning with _345 display but not my sample image.

Apple.com/feedback to give feedback.

When searching for text, we have currently to search for complete words - the search string will only be found , if it is appearing at the beginning of a word, or following an underscore or another separation character, see: Changes to Searching for Text in Photos 5… - Apple Community

There has not been any improvement after the Big Sur update.

And recommended by OT, I also prefer to use smart albums to search for text, but even then the text has to appear after a separation character.

Partial searches work in a Smart Album:

Try using a smart album to do the searching:

That was my bad. I was using title, not filename