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How does this work on a technical level?

Does it framebuffer grab the current session and inject input events?

Usually the way to be able to even get the login screen is to use DRM/KMS based capture and use uinput for device emulation. It doesn't strictly need root but it does need privileges a user wouldn't have, like access to the DRM card nodes and uinput device. Sunshine does it this way.

It entirely sidesteps the problem of X vs Wayland, although this does mean the unattended physical seat can be physically taken over while you are remote.

As I remember, generic DRM interface allows only writing to a framebuffer and not reading from someone's else framebuffer. Do they use some vendor-specific APIs?

Not that I'm aware of. I'm on my phone right now but any KMS driver that supports KMS and dma-bufs should work.

The basic idea is to enumerate the CRTCs to get the relevant framebuffer plane resources and then use dma-buf exports to pass them to an encoder. Reality can be a bit more complex due to the existence of overlay planes but that's the idea.

https://github.com/LizardByte/Sunshine/blob/master/src/platf...

Yea I need to know this before I even consider something like this. I'm guessing it's a daemon that runs as root in the background.

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