Home Assistant OS is one of four different ways to install Home Assistant. The table in this post explains the major differences between them:
The main difference is that Home Assistant OS includes everything including a streamlined operating system. It’s not a general purpose operating system but customized to require very few resources and to run on specific hardware platforms. That means it won’t run on just any machine but only the ones it supports (RPi and Intel NUC).
If you’re like me and want to use an ancient laptop, you will need to use Home Assistant Supervised. You must supply the operating system: Debian Linux.
As for SmartThings, have a look at the integration for it:
As for Node-Red, it’s available as an Add-On (Add-Ons are supported by Home Assistant OS and Home Assistant Supervised). Installation is a matter of a few clicks.
Overlooked to say: Welcome back!