Where to start?

I want to develop sensors using zigbee connecting to Hubitat, but don’t really know where to start.
I have some programming experience from long ago so aren’t starting from scratch, but have never delved into Hubitat, drivers or zigbee.
I thought I would start with something to measure a voltage and return it to the Hubitat to include in an automation. So I can turn on a battery charger when a lead acid battery starts to go flat. Not massively useful, but should teach me about the general principles.
I’ve searched YouTube and articles, but can’t find a starting point.
Any suggestions?