Working with importet Images in the cataloge

To add to DJs comment, when you add new photos, be sure to provide a unique keyword to all of the images (for example: "Yosemite," or "Aunt Maude's Birthday" and you can then quickly and easily find them. Alternatively you can create a new folder for each import. Or, you can do some sort of combination of the two. 

All of your images are logged in your catalog, it's how you prepare and organize them is how you find them.

They're not gone. Click on "All Photographs", it's under "Catalog".

Please spend some time learning the basics of how Lightroom Classic works. A free beginner's e-book about using Lightroom Classic is here, and it's free (in case I didn't mention that), costs you nothing, and it is loaded with helpful information. And it's free.