First, nice photo! Even "old" gear can take great photos. Throw motion and/or low light (with a fast lens) into the mix and you'll beat a modern smartphone.
The quick lead into the exposure triangle is:
ISO is basically "gain" applied to the photons that hit the sensor. Some gain = fine. More gain = you start to run into signal to noise ratio challenges
shutter speed helps you freeze the action, or can also let the action blur on purpose. Examples of intentional blur include panning photos (think auto racing) and long exposures (at night or during the day with the aid of a ND filter)
aperture. This is the ratio of focal length to lens aperture. Keep in mind it's 1/x, so as x grows the actual aperture is getting "stopped down" (aka closed/smaller). Wider aperture (aka small denominator) = less depth of field and more light will hit the sensor. Stopping down = more depth of field and generally more sharpness/less vignetting, but if you take this too far you'll hit diffraction and lose sharpness
You wind up trading values against each other in various scenarios, which is why it's called the exposure triangle. It's very much a "you pick two and deal with the third" situation. Which two you prioritize really comes down to what you're trying to accomplish.
For your barn photo's exposures, let's talk tradeoffs. It sounds like you know that your ISO value was too high, especially for a static subject and good light. So how to get it to go down? You could do a mix of:
using a slower shutter speed. Unless you have a tremor, the rule of thumb is minimum shutter speed should be more than 1/focal length. You could have easily shot this at 1/100, if not lower. That would cut ISO down to around 1600
open your aperture. f/14 is very closed and likely isn't needed unless you really want to see something deep in the background/foreground. You're also likely losing some sharpness due to diffraction