Big picture is that Home Assistant is an open source (free) home automation platform with hundreds of developer / users contributing code and integrations. I currently use it only for data acquisition and visualization, but not yet for automation (mostly do that in Apple HomeKit). I like HomeKit because it has so many off-the-shelf, ready to go integrations. A few samples:
Shelly and ESPHome offer a wide range of connected sensors, though I use the Sense, Tesla EV, RainForest Automation (Eagle 200) and Ecobee hardware and integrations to pull together all the energy data from my house. You’ll also notice that it is also a meta-platform and can ride on top of HomeKit and SmartThings integration.
Home Assistant Integrations are pre-built to automatically identify and walk you through connecting to each of the platforms / sensors you want to integrate. Once it integrates Home Assistant automatically samples data from all your sources depending on the configuration of each integration (sampling rate is configurable, but defaults are usually pretty good). Home Assistant stores incoming data as entities carrying sensor “state information” and associated attributes over time.
As Home Assistant brings the data in, you can also create calculated state based on functions of the incoming data. Here’s an example where the Tesla EV integration offered up voltage and current, but not instantaneous power and I had to create it.