Regarding tutorials, there was a post by unitelabs 5 month ago (links are there) Introducing the Connector-Factory: a python template for kickstarting device integration with UniteLabs CDK. The slightly older (started 2021) has code and docs here: https://gitlab.com/SiLA2/sila_python/.
Yes (its in the linked tutorials). There are implementations for other programming languages but they should work similar.
You already categorized it nicely ;-). From python-API (UFactory), over more (Hettich Rotanta) to less (Thermo-Cytomat, kbiosystems) complicated serial interfaces and nice .dll (VWorks, ThermoMoverFramework) to ancient activeX (labcyte) and 32-bit, licenced .dll (Biotek) you rather reverse engineer its a colourful mix.
I think, PyLabRobots makes it easier to interface supported devices from python (I tried it a year ago with an OT2).
How the command is sent from server to machine is not SiLA`s concern. Its only about the server-client communication (user is client, device is server).
I have servers from my current lab here https://gitlab.com/OpenLabAutomation/device-integration, some older ones are hidden here: https://gitlab.com/opensourcelab/devices. Not mine: https://gitlab.com/sila-driver-group.
I agree about the sila2-website. I had not looked on it in year, but servers there are definitely not free. Seems like equicon has ramped up implementing them.