You can, sort of, insert Google Drawings in Slides.
I am a fan of Google Drawing. While it does not have all of the features of Adobe Illustrator, I am able to create quite a bit with it. What I especially like about Google Draw is that it is collaborative and the drawings are in my Google Drive with the rest of my documents.
Unfortunately, Slides do not allow for inserting a Google Drawing. You are not able to simply access your Google Drive and insert the drawings you already have.
In your Google Slides presentation go to the insert menu and choose image.
This will insert your Google Drawing as an image. The inserted image can be resized if needed. Updates to the Google Drawing will NOT update the image in the Slides. Inserting by URL creates a copy of the image inside of Slides.
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Notice in the Google Drawing poster there are images of QR codes. If you would like to add QR codes to your Google Drawings for Google Slides you can do this in Google Chrome! Go to any website that you want to site in the poster. In the address bar click on the icon of 4 squares with a broken square around this. It is only visible when you click on the address bar. Download the QR code and DRAG it onto your Google Drawing.