If you mean the Blue, Yellow, Green… those are different pieces of hardware that have been produced to run Home Assistant on. If you plan on using a NUC for you HA server, you don’t need to worry about that at all.
Yes and No.
Home Assistant can likely integrate with and control many, if not all, the end devices you are using with Google Home.
While there are a number of devices that can be used as voice inputs for Home Assistant, the first official voice hardware is not available yet (release is scheduled for Dec 19, 2024). Regarding the existing options on their own… they are not plug-and-play, 1-to-1 replacements for a Google Home or Alexa device.
They will work for day-to-day control of many of your Home Assistant-controlled smart devices.
Some, but not all, are suitable for use as media players for audio.
They will not answer random questions like their cloud-based commercial counterparts. By connecting to one of the available LLM models/services it is possible to receive those kinds of answers; but it is not available out-of-the-box, not super easy, and suffers from AI bullshitting.