How would you build copy and paste across computers?

There is https://github.com/jedisct1/piknik and similar open source projects that implement their version of cross-machine copy/paste as a command-line program. There is https://symless.com/synergy, https://github.com/debauchee/barrier, and https://kdeconnect.kde.org/ that can synchronize your GUI clipboard. Browse GitHub topics (repository tags) like "clipboard", "shared-clipboard", and "clipboard-sync" to explore more options.

I use an NFS share to transfer files and keep work in one central location, but a true copy-paste store across devices would be interesting. You'd have to be able to sync the clipboards across many devices, so to I'd either propogate updates in a similar way to routing protocol daemons, or go with the less flexible but easier to set up clipboard daemon->ssh-and-xclip. It would probably be wise to use a VPN like wireguard to maintain some level of security and confidentiality.

I send myself a message on whatever app I have logged in on both machines. Discord functions as a sort of notebook for me, it's quick to open on phone and I've usually got it running on laptop.

Mac’s have this already - you can even copy and paste between your iPhone and computer. It’s a feature that not many people know about. I only discovered it by accident.

One way if you seldom do it: Compose new email in gmail and it appears in the draft. Go to the other computer and you see it.

There are already services that do this. I use PastePal and it works fine.

Sneakernet is the simplest thing that might work.

https://en.m.wikipedia.org/wiki/Sneakernet

Copy -> write to a secure remote server that only allows the value to be accessed once -> read it from other computer and add to clipboard

I send myself a message on Telegram in Favorites. There is a lot of memory. And files are sent without quality loss

i have a NFS drive shared on my LAN which rsync to GoogleDrive.... works as a charm.

Parsec