Long story short - Install TrueNAS to a small SSD or a flash drive (less optimal). Whatever storage the OS is installed to is used only for the OS and nothing else, so for example installing TrueNAS to your bulk storage HDDs is going to stop you from being able to save anything to them in TrueNAS. Once you do that, you can create a "pool" of disks out of your hard drives for bulk storage. You can run a single disk, if you have two disks you can make a "mirror" that way if one fails, you can replace it and it will copy your data back over from the other one that is still live, but you only get the total storage of one single HDD that way. If you have 3 or more then you can start considering RAIDZ pools which will combine those HDDs into one big storage target and you can designate 1, 2, or 3 of those HDDs as "parity" so that similarly to the mirror, even if one or multiple fail (to a certain extent) you can keep using everything and replace the failed drive and have it get back to a healthy state.
That's a gross oversimplification but an insight at least as to the direction you probably want to start heading down. There's a TON of very important or very helpful information to be read in their documentation. Not all of it will be helpful immediately, or at all, but at the very least keep it handy.
https://www.truenas.com/docs/scale/25.10/gettingstarted/
A few words of wisdom, until you are reasonably confident in your understanding of what you are doing with it, avoid storing anything on the NAS that you can't afford to lose, and it is always VERY STRONGLY ADVISED to follow the 3-2-1 backup strategy:
3 separate copies of your data, saved on...
2 different storage mediums (such as your NAS (1) and an external harddrive (2)), with...
1 copy kept off-site (store that external hard drive in a safe deposit box at a bank and only grab it when you need to refresh your backed-up data or recover your data, or additionally back up your data (or at least the really important stuff that's irreplacable) on Google Drive, Dropbox, OneDrive, etc)
That gives you a VERY high chance of being able to recover from your NAS totally dying from a lightning strike or all of your HDDs dying in quick succession or some other sort of catastrophic failure.
Edit - oh yeah - Glad to hear it started working!!