how do I migrate or transfer Aperture pics to Photos

Well this is not a simple task, and it does depend on what you mean by "merge"... All the official documentation suggests that you can't do it at all, and instead you need to export from one library to the Finder and import to the other library. This of course breaks the link between originals and edited versions, also forces you to choose between losing work because you choose originals, or losing the originals because you choose edited version or having massive duplication. You'll also lose face recognition data, smart albums and other things.

There is a very long-winded method using the iCloud Photo Library but that can take weeks, but here are some links that explore how that might work:

https://mbsdirect.com/mbs-blog/article-need-to-merge-photos-libraries-here-are-your-options

https://blog.macsales.com/40866-rocket-yard-guide-combine-multiple-photos-libraries-into-a-single-library/

These are just some of the reasons why Photos is in no way a replacement for Aperture, but then who replaces an $80 app with a freebie?

If you still have Aperture, I would merge all your Aperture (or iPhoto Libraries) using Aperture, before you migrate them to Photos. Aperture at least can merge libraries as libraries.

Aperture 3.3: How to use Aperture to merge iPhoto libraries - Apple Support

And use Aperture's batch changing tool to write all metadata into the originals. Then it will matter less, if you only export the originals from the exported Aperture Library.

I am sorry to say, the adjustments applied in Aperture will not be migrated well anyway to Photos. Most of the adjustments will just be migrated as a second quasi-master, with the adjustments burned in. If you want to add further adjustments, you will have to revert the adjusted photo to the original. And the quality of the migrated adjustments is sometimes horrible, i.e. the Aperture color adjustments. Many of them will result in glaring colors after the migration.

For example: In Aperture I touched up the red color of the poppy blosoms to make them stand out. The effect in Aperture has been discreet and the photo still looked natural. When I open the library in Photos, the contrast of all colors has been exaggerated. I had to revert most of the Aperture edits after the migration to Photos and recreate the adjustments.

Very simple: Open the Aperture library with Photos.

Hold down the option (or alt) key key and launch Photos. From the resulting menu select 'Choose Library' and select the Aperture Library.

None at all. You'll just have a second library.

Are you thinking of trying to merge them into one?