LocalSend is an open source peer-to-peer service that will copy files across devices that are connected to the same network.
After installing the client on Windows, macOS, Linux, Android, or iOS devices, the program lets you select files to transfer from one device to another.
The transfer is end-to-end encrypted, with the data never leaving the local network. Devices can be configured to require a PIN and affirmative approval before the file transfer takes place, or configured to auto accept any transfers from approved devices.
I used to use Syncthing for this, but Syncthing was always a bit too fiddly and would fail frequently. Additionally, I tend to move hundreds of gigabytes every week, and sending it out to the Internet from the sending device and then back down over the Internet to the receiving device was much slower.
The tradeoff is that LocalSend won’t let me send files from my phone to my desktop from different networks like Syncthing allows.
But for what I need—moving files from multiple devices to a central device or vice versa—LocalSend just works.