Grogster Don't feel overwhelmed -- if you're just now expanding your horizons beyond Windoze and looking for KISS introduction to the open-source world: OMV is a great place to start! (I'm not a beneficiary in any way of the OMV project; I'm just a personal tech enthusiast that enjoys quality products where I find them i.e. OMV).
I believe everything you wish to accomplish is comfortably provided within the web interface; I don't expect you'll run into any common tasks that will force you back to a command-line once it's up-and-running.
The "KISS" rule for user/permisisons management for all IT systems (Windoze, Linux, email, network services, etc.) is always:
1) Groups first
2) Users second; assigned to appropriate groups
3) Assign roles third; by groups when possible (unless unnecessary)
There's nothing wrong with only setting up one user if it's a home network and you're the only user... just skip step 1! To crate a personal user in the web portal just click 'Users' => 'Users' and the "plus" symbol and complete the form. Always remember to click the check mark in the yellow banner with OMV! (New settings entered in the web portal aren't always applied to the system until you do.)
Next go to 'Services' => 'SMB/CIFS' => 'Shares'; this is where you configure service-level options for the network file share service (these settings are only necessary for existing shares if you don't specify these options when creating new shares.) Set "Public" to 'No' -- this is the big divergence from Chris' video for shares to be Guest acessible. Also "Browsable" to 'Yes" only for the folders you want to be listed when navigating to the NAS' address in your file browser... Folders that are not "Browsable" can still be accessed by their full file path but will not be displayed in your file browser.
Lastly go back to 'Storage' => 'Shared Folders' [no doubt where you started haha] -- here, you can click one share at a time, and click the icon above that looks like a folder with a key on it labeled "Permissions". Here you can set "Read/Write", "Read-Only", and "No Access" as appropriate for groups [or users; more likely in your case] to restrict access to each respective share on your NAS.
Et voila: hopefully that will accomplish the necessary user-based security you're seeking for your NAS! Again don't forget to always click the check mark in the yellow banner when it appears following each configuration change 😜