Sure, I’d be happy to explain. Essentially, Google Analytics uses a system of first party cookies. These cookies hold onto an ID that tags the user’s browser while they’re on your website, right? Unfortunately, these can’t be directly used in your Data Management Platform (DMP).
Moving or copying these cookie IDs to your DMP isn’t impossible though. Here’s what you can do: You must wait for the same user to return to your site. Once they’re on your site again, their browser will be recognized and you can then transfer their ID to your DMP cookie.
You also have another option. You could write your DMP’s userID as a custom dimension in Google Analytics. This will give you a way to build a bridge or link so to speak, between the data you have on GA and the data on your DMP.
Hope this makes more sense! Do let me know if you need any more help.