2018 MacBook Airs and MacBook Pros can run Sonoma – the current version of macOS – but I wouldn't count upon them being able to run the next version of macOS. They're right at the cutoff point now.
If you want "all the info" about specs, I would suggest that you look at the Technical Specifications on Apple's site, download MacTracker (a third-party app with versions for Macs, iPhones, and iPads), etc.
Keep in mind that
- RAM is soldered in on these Macs
- The OWC Web site doesn't list any internal SSD upgrades for 2018 MacBook Airs or MacBook Pros – which very strongly suggests that no upgrades are available
If you purchase a used Mac, you should make sure that the seller has prepared it as outlined here:
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
Otherwise you may run into problems using the machine. For example, I believe that all of the 2018 MacBook Airs, and all of the 2018 MacBook Pros, have the T2 Security chip and therefore the ability to support Activation Lock. If you bought a used Mac for a price "too good to be true" and later discovered that it was Activation Locked to some person whose credentials you did not know, and who would not help you,, you could have a very expensive "brick".
This article is about iPads and iPhones, but some of the advice applies to Macs as well.
The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community