Open up two docs in preview. Let's say Doc A is the one you want to merge into. Open Thumbnail view in each doc. Select pages in Doc B you want to merge over to Doc A (some or all). Drag and drop into thumbnails view in Doc A. Reorder as desired. Profit.
Been a while since I used it but IIRC you should be able to show the page thumbnails sidebar in each document and drag them from one window into another.
Probably also accessible via Preview’s Automator/Shortcuts actions if it’s something you need to do frequently.
Copy a selection of pages from an existing PDF, choose File > New from clipboard. From that point you can edit the pages in the PDF using the thumbnails sidebar and drag pages from other PDFs into the bar.
As I just posted elsewhere, I do find I sometimes have to copy a PDF before it'll let me do this to it. I assume it's some kind of flag that isn't preserved through the copy and there's actually a better way to fix it, but my work-around is simple enough that I've never bothered to figure out what it is.
