How can I find the year of these Giant XTC 2 bikes?

bikepedia.com has a lot of data, sometimes a bit incomplete. https://www.bicyclebluebook.com/ is my other quick-n-dirty go-to site. Take the values with a grain of salt, since the virus-caused biking craze has driven up prices on everything bike-related.

Looks like those bikes might be 2008 or 2009 Giant XTC2s, although truvativ is listed as the stock crankset, and the color pattern seems to be the inverse of the one listed. I've noticed in these refs that alternate color patterns are often not listed, and bike shops often swap in/out parts on new bikes to fit customer preferences. The stock rims are listed as WTB in 2008 and Mavic in 2009.

They look good to me -- solid entry-level hardtails; what would you try to achieve with "upgrading" versus just making sure the drivetrain etc components are functional and in good shape?

(based on updated info from OP) The Dart 3 IS a basic entry level fork (but not an even cheaper "fork shaped object") and IIRC uses a coil spring rather than a fancier and much more expensive system like an "air shock". You would spend many hundreds of dollars to move up to a significantly better fork; at this point, I would just ride the bikes. You'll learn how to do basic maintenance, and certainly identify features/quirks of the bike that you do like, and some you don't. 6 months or a year down the road, you'll know what type of riding you like, and have an idea of what bike features will support that. Then you'll be able to make an intelligent decision on what to do with these bikes or perhaps get another.

Here's what I would do:

1. Certainly get a new chain; keep the old one to line up with the new one so you can shorten the new one to the correct length.
2. Clean off the front derailleur (and rest of bike), but FD function is usually not affected by surface rust.
3. Have someone with knowledge evaluate whether the front chainrings and rear gear cassette are worn and need replacing along with the chain. Have them evaluate whether the shifter and/or brake cables need replacement. Have them evaluate whether the tires are safe or need replacing. Have them evaluate whether the wheels are safe or need fixing. This is an excellent opportunity to find a Local Bike Shop (LBS) that you can trust with questions and fixes beyond your knowledge. A "bike tune up" should cover most of these questions, but replacement parts will be extra.
4. Get grips, pedals and a saddle that you like. They're all straightforward to install, but getting the old pedals removed and new ones installed is much easier with a big "pedal wrench" which will run $25 or so.

Books I've found helpful and easier to use next to the bike than a tablet: "Zinn and the art of Mountain Bike Maintenance" You can save a lot of money by getting the 2010 (earlier) edition, which still covers all the technology in your 2008/2009 bikes. "The Bicycling Guide to Complete Bike Maintenance" by Downs (also cheaper as the previous edition)