Welcome!
What does cloning mean in mod creation?
Values in the Attribute Editor are generally stored in XML files - one for each unit, ability, weapon and so forth. A "Tuning Pack", as Relic calls it, replaces some of the stock XML files with custom ones to modify values in the game. You can either make a completely new one that is mostly empty and fill in all the values (quite a lot, usually), or copy an existing one and modify it, which is usually much less work. That is called cloning. When cloning, you have the choice of keeping the file's name, which means that your mod will overwrite the original one, or changing it, which means that your mod will add something new.
How do you adjust tank armor values? Is that under the attribute editor?
Yes. In the "ebps" category, look for your unit, clone it while keeping the name and look for "armor" in its "health_ext".
How do you adjust the rate of fire and accuracy for vehicles? Is that also under the attribute editor?
Yes. In the "ebps" category, look for your unit, open its "combat_ext", choose the weapon (most vehicles have more than one), then right-click and "Go To Reference", then clone that weapon while keeping its name and look at "accuracy" and "reload".
How do I make a mod with unlimited heavy tanks? Is that under another portion of the modding tools besides the attribute editor?
That's in the AE as well. In the "abilities" category, look for the ability that calls in the tank. Clone it while keeping the name and open its "requirements". They should all have a "required_not" containing a "required_entity"; delete the "required_not".
Modding CoH2 is not as difficult as you think. Minor "balance changes" are pretty easy to achieve with a bit of basic knowledge! I recommend watching these tutorials:
https://www.youtube.com/watch?v=vTr_I_DSB7w
https://www.youtube.com/watch?v=S6mdO-FQ680
https://www.youtube.com/watch?v=wsflS3R0uEo
https://www.youtube.com/watch?v=jvnKsZ0pzG0