Q: Where are photo file names?

correction.  I misread your questions.  You can change the titles but not the filename of the original files in the library.  You would need Aperture for this.

Only when you export - in the export use the option to use title for file name

You can not make any changes to the original photo including the file name while it is in Photos

LN

Ahh... I had not realised that, thx. Now I see the issue. BUT - where is the Info panel? I think in iPhoto you could click it on under View/ but I don't see it in Photo's, it doesn't come up consequently I have no idea about any of the info pertaining to my Photo's (including Filename).

where is the Info panel?

It's a small floating pane that you bring up with the Command + i key combination.

You can also launch the Info panel from the Window menu in Photos:

Window > Info ⌘I

The idea that you have to Get Info for every picture is unrealistic, it's cumbersome and time consuming.

And what happened to Search?! Using Get Info, I copy the filename into the Search panel and NOTHING.

How useful is that? Not at all, actually a big fat problem.

Personally I find Photos uninutitive and frustrating. It feels like a giant box of pictures that are a complete mess. I may add a photo from 2013 in 2016 and I can't add it to the folder with the others from the same event unless I want to re-create an album which I had in iPhoto but Photos didn't import.

What a mess. Apple please stop "fixing" things that aren't broken until you can actually do it right.

The idea that you have to Get Info for every picture is unrealistic, it's cumbersome and time consuming.

If you keep the Info panel open, you just have to select a different photo and the Info panel will display the metadata. just keep it open.

Using Get Info, I copy the filename into the Search panel and NOTHING.

That should not happen. There is something wrong.

If an image is in your library, search should find it by the filename.

For example, searching for IMG_6547.JPG produced the JPG file:

Exactly, it should, but it doesn't:

Try repairing the library according to the instructions in this Apple document, Repair Your Library, and try the search again.

Warning: If you use iCloud Photo Library with Photos, this action will cause Photos to upload all your pictures again (from Fix missing or incorrect thumbnails in Photos - Apple Support).

Exactly, it should, but it doesn't:

I have seen this happen too. Sometimes the search does not find items that are clearly there. In that case I make sure, I have the "All Photos" view selected when searching, and I clear the search field and repeat the search.

Hidden photos will not show when searching from the "Photos" view, but they will show, when searching from the "All Photos" view.

Shortsighted of Apple to not display file names in Photos app. Many of us have given searchable file names to thousands of photos over many years.

Finally I have located an app to display file names below thumbnails in Photos app: Documents 5 by Readdle. Good app to do some file management and read your files on ios devices. Integrates with Dropbox well.

That will not help in Photos on a Mac - and this discussion is about displaying the filenames below photos on a Mac.  Power Photos can display several kinds of metadata below photo thumbnail on a Mac.

Shortsighted of Apple to not display file names in Photos app. Many of us have given searchable file names to thousands of photos over many years.

Finally I have located an app to display file names below thumbnails in Photos app: Documents 5 by Readdle. Good app to do some file management and read your files on ios devices. Integrates with Dropbox well.

Correct. While on Mac I just look at the file names on iPhone or iPad.

Otherwise seem to be stuck using Command-i individually for each photo. Apple hasn't provided an option for displaying file names below photos on Mac.

Haven't looked at Power Photos.

but there appears to be no option in Photos to show the filename. So none of my "titles" appear. Am I missing something?

No, you're not missing anything.  However, there is a way to get the file names to be displayed in the Title field underneath the thumbnails.  Here's how:

1 - use the View ➙ Metadata menu option and check all of the metadata options.

2 - select the photo you want to have the file name entered into the Title field (don't do the entire library at once as the script could time out) and run ones of these two Applescripts from the Photos for Mac  User Tips: 

Script: Batch Changing the Titles to the Filename w/Extension

Script: Changing the Title to the Filename without Extension

You can download a compiled version of the script as an application from this tutorial site: P01 - Applescripts from Photos’ User Tips Compiled as Applications

BOU