I use Apple Photos to edit. I do not understand how to save my photos at high resolution to print them out.

How are you saving the edited photos in Photos?

JPEgs are a format with a lossy compression. When you save edited photos a JPEGs, it is important to specify, that the compression should be lossless.

Select the edited photo, use the command "File > Export > Export 1 Photo", then set the options for the exported JPEG like this, to get the highest possible quality:

Is that, how you have been exporting the edited version?

What are the dimensions (width and height in pixels) of the exported file?

To print the photo at 16” X 20" prints you should ensure that the pixel size remains the original 5760 x 3840 pixels. The export option "Size > Full size" should ensure this.

The file size may be less when exporting as a JPEG, but the resolution should remain the same, unless you cropped the image. Ignore the "Dpi" tag embedded in the photo. Photos will always embed 70dpi on export, meant for displaying it on a screen, but you can still print the image at 300dpi.