@tom-muscleverse There are a lot of really knowledgeable folks in the Onion community, we've all struggled to kick off our projects so don't be afraid to ask questions. Of course it's great to have another Aussie on board
The Omega2 runs OpenWrt which is basically Linux minimised, so a lot of the concepts you want to understand about your Omega2 can be found googling Linux stuff. Now to each of your questions.
Questions 1 and 2 can be answered together. There are 2 main processes we use to deploy a customised version of the Omega2 firmware. 1) You can use the build system to roll your own version of the firmware; 2) Take an Omega2 and install/remove the required/undesired packages, add your scripts and configurations etc, once you have that device setup as you want to mass deploy, you clone that device to all of the new devices. Each of the two options have their pros and cons which will summarise here.
Option 1, roll your own firmware: New users without software development experience can find this a bit frustrating sorting out the various dependencies and error messages you need to resolve depending on the machine you are using to build this, the upside is that once you have this up and running you can configure the entire system and add scripts and files, then build a deployable firmware image, specific to your requirements. Importantly a factory reset will reset to your firmware build.
Onion provide a docker image to get you up and running with your build system quickly. Personally I prefer to install the build system myself. You use the build system on your PC, not on the Omega2 as IoT devices don't have enough capacity nor juice to build this stuff. Then you transfer your firmware to the Omega2 and run the firmware upgrade command: sysupgrade
Option 2: Set up a single Omega2 as you require, then clone it to multiple devices. Easier to do, but a factory reset will blow away your firmware and install the factory firmware. @Lazar-Demin wrote a great article on how to do this:
Also look at this thread for some good info:
Setting up a build system is documented here: https://docs.onion.io/omega2-docs/cross-compiling.html
I would suggest reading the whole document as it's very well written and provides a great understanding the Omega2 device.
With regard to question 3. Usernames are in /etc/passwd and their passwords are hashed and stored in /etc/shadow , just like *nix, google shadow password for a more detailed explanation of how and why *nix works this way. You use the command passwd to change your password, don't edit these files.