There is a terrific command you already have (accessible by the terminal console) called “rsync”. Been around for decades. Read the “manual” for how to use by issuing “man rsynch” command. You’ll find lots of advise via searching internet.
Alternatively you could purchase and use Chronosync. Provides a user-interface to setup syncs, scheduling, etc.
A. rsync only does one-way sync. I’m assuming the OP was looking for something that would keep both computers in sync, not just one way. Doing two-way sync with rsync is technically possible, but more complicated. Also, it’s a command line tool, with a ton of configuration options, many of which are confusing. I say this as someone who has used rsync for many years and still does not fully understand some of the options.
B. There are two kinds of people in the world: 1) people who have accidentally deleted files with rsync and 2) people who have never used rsync. It’s just a thing that will happen if you use rsync long enough. Casually suggesting rsync to someone is potentially very dangerous. Not to mention that I don’t think it will do what he wants.
C. There is another tool called Unison which is somewhat like rsync but not as complex/dangerous and does two-way sync, which makes it a much better recommendation if someone insists on a free, command-line based solution.
D. Having said all of that I would still 100% implore you to try Resilio Sync and/or ChronoSync.
With Resilio Sync you can sync your Desktop folder between two computers, or your Downloads, or both. You can pause individual folders. You can sync 3-4-5 computers just as easily as 2 computers. There’s no online “cloud” component so there’s no subscription (there is an initial cost, but it’s one-time, and there’s a demo so you can try it first).
Also, with the recent 2.7.0 release, you can sync two folders on the same computer, which can also come in handy.
If you have iCloud Storage, and are using the same Apple ID on both devices, your data should be synced.
Could Storage Solutions: I would put he Downloads inside of Dropbox, Box or Google Drive and it can be synced. Most Cloud Storage solutions will give this feature. But you will need cloud storage subscription depending on how much data you have.
Rsync and Chronosync: Like everyone suggested are good if you don’t pay for any cloud storage.
If you only need to sync in one way and don’t do that too often, then you can just copy Downloads from one Mac to a SD Card/usb drive and load it on another device.
I’ve used rsync to do two-way syncing. It scared the hell out of me then and the thought of doing it again scares the hell out of me now
(It’s not that bad, but I’ll second your advice that it’s probably a bad idea for the OP to jump in with data that they care about. I might be a fun project to play with, with some directory trees that contain dummy data just as a “fun” exercise (for some small value of fun))
I do most definitely recommend just trying it (though not with production data at first); I’ve used various flavours of Unix in my day to day work since the early 90s.
It (the OS family, and more importantly, the design philosophy) rewards devotees with power, flexibility, and automation beyond the wildest imaginings of people who never venture into that world. But it also rewards the less than devoted with opportunities for data loss and downtime beyond their own wild imaginings.
I’ve witnessed a lot of the latter in my work and so tend to be a bit more cautious when recommending that path to people who seem to be asking for a “simple” solution.
It’s probably worth considering the cost of “free” solutions too: The time involved in learning and upkeep is a valuable commodity that often outweighs the cost of purchasing someone’s ready made solution.
tl;dr I’m naturally inclined to your way of thinking, but I’ve seen that go wrong a lot of times too.