how can I access to my photos located in external hard drive via Photos app and Windows OS (when I need it)

For a lot of reasons: Photos is not the app for you.

Photos formats - jpeg. tiff etc - are OS agnostic. They're the same on every OS. You don't have to use Windows to share with Windows users.

Photos is not a photo viewer, it's a manager and parametric editor. It's built around a database, and like any database it can only work with data that has been imported to it. If you use Photos in the default configuration it will copy the files into the database - but that means you will be duplicating, and you don't want that. The other possibility is that you use Photos in referenced mode, which will not copy the files. But Photos is very unreliable in that mode, and is missing most of the tools you would need. Don't do it.

So, if you want a photo viewer for Mac then check the App Store, but Photos is not the app for your usage scenario.

While I understand why Photos looks tempting, don't bend yourself all out of shape to use it. It's fine at what it does but there are far more powerful apps in the same field, many of which are cross platform.

If you use Photos in the default setting then it will double the space used.

They are as secure in the database as they are now in the Finder. Of course, you do need to back up important data.

Photos has no cross platform capability. But yes, when you export you export to standard photo formats, like jpeg, tif, png and so on. They can be reached without export, for instance, they can be easily shared to other apps, like emails or word processors. But they cannot be accessed from a Windows box without first exporting.

You can have multiple libraries in Photos, but only one can be designated the System Photo Library, and only the contents of that cab be shared across devices.

I would suggest that you explore two apps, both have cross platform ability, and both are more capable than Photos - though they do require payment (doesn't everything have a downside :) )

Mylio and Adobe Lightroom. Mylio would work with your current images on that external. Lightroom would upload them all to the Cloud, and then make them available wherever you want.