How to “sync” a laptop to an external hard drive if the hard drive is larger than the laptop drive?

I have a lot of data (TBs) on Dropbox. My laptop hard drive has only 500 GB. For the last few years, I’ve been using the sync feature on Dropbox and only downloading what I need for my laptop as needed. So, at any given time, I only have 500 GB of data locally, with the rest “on the cloud”. Goal: I want to stop using Dropbox. I do want to have a subset of my documents available locally on my Mac and the rest retrievable “on cold storage” when needed and connected. Plan: Make a copy of all my Dropbox files on a 15 TB external hard drive and stop paying for Dropbox storage. I want all my data to sit on a 15 TB hard drive and then pull in a small subset “local copy” to my laptop as needed. When I plug in the external hard drive, all the changes made on my laptop will be mirrored on the external. Problem: Can I sync up the hard drive with my laptop to make the hard drive function like Dropbox does?