Sorry for ignorance. So I just got the ras pi 4 as I’m building my smart home network. Just finished major home reno and have all my cables ran. Is HA the ‘controller’ or I need to buy this piece of hardware? Thanks.

Define controller in your own words.

HA is software that runs on your raspberry Pi.
It connects to devices in your house and can control them if you set up automations.
Some people, myself included, use Node-Red to control devices.
Node-Red can be installed right in HA under Supervisor.

Simple answer: Yes. HA will be the controller of your home automation. Use the RPi 4 to start with

Thanks look forward to getting it up and running soon and the help from community. Seem like some very nice, friendly people.

The brains of the systems. Point everything filters in/out of.