I have several folders and files that I would like to link to a local folder that happens to be a Dropbox folder. Specifically I'd like to put Finder aliases to some files and some folders into a folder that is on Dropbox. What happens when I do this? What is synced to Dropbox? Is it just the Finder alias itself (a special kind of file only meaningful on macOS) or are the linked-to files or folders copied to Dropbox?
The Finder alias will appear as a zero-byte file in Dropbox. Any files linked to it will not be uploaded.
If you would like to have Dropbox sync the contents of the alias, you can instead use what's called a symbolic link. Basically, it's an alias that the operating system (and Dropbox) treat as indistinguishable from the actual folder. This question on SuperUser answers how to create a symbolic link.
I tried putting an alias for my Documents folder in Dropbox (before macOS Sierra put the Documents folder in iCloud), and it didn't work. If I went to my Dropbox account online, there was a folder called "Documents," but it never synced any of the content.
Be super careful about using that app, though. Without thinking, I put my Pictures folder in Dropbox - the Pictures folder contains Photos.app data. Every time I added a photo to Photos.app, it had to update Dropbox. And with thousands of photos, that file was at least 12Gbs. It took forever and caused horrible lag.
Just in case anyone finds themselves in a similar situation, to fix this, I disabled Dropbox syncing then deleted the Pictures folder from Dropbox.com. This did not delete any of my photos on my computer and stopped the insane syncing problem.