There arn’t really “commercial” airways (this info is USA focused, internationally things change slightly depending on where you go), just ones above FL180 (like J and Q) and ones below (like V)
There’s nothing stopping anyone in say a private jet from following a commercial jet (as long as they maintain separation, they can’t follow super close).
Lets take a look at a recent flight of Stevo1Kinevo (or at least his usual plane)
He flew from 0A9 to KOPF via SUG SPA CRG OMN MLB BLUFI . It doesn’t directly list any airways in his filed flight plan, but when you chart it out, on say skyvector or navigraph charts. He actually follows airways for large parts of his route.
the path from 0A9 to SUG pretty much joins with the airway V35 (the V means it’s a low altitude airway based on a VOR radial), then goes direct to spa, which overlaps with V53. The long leg to CRG doesn’t match up with anything as far as I can tell, but then CRG to OMN then MLB all matches up with some J and Q airways (high altitude airways, with J being based on VOR radials and Q being RNAV (mostly GPS, but can be other systems) based) that also have V airways underneath.
Did he directly follow these airways? No, but you can clearly see how he used them to plan his route.
Start and end of his route
Another example, a private jet who flew into the same airport. Until he got near florida and the gulf of mexico, he mostly just went from point to point not following or mirroring airways, but once he got down there, he clearly (as listed in his flight plan) flew a J airway until he joined a STAR to get to the airport.
Basically… to make a realistic flightplan use fixes, but you don’t HAVE to use airways (but you sure can!) , and do try to use SIDS and STARS ( STARS in particular are nice because usually tell you what height to be at at their waypoints, doing some of your descent planning for you), but something else to lookout for is that some SIDS and STARS are intended for specific types of aircraft, that CYY1 star that jet flew into OPF is only available to jets (basically read the charts before deciding any sid or star is right for your flight)
You can generally copy flight plans off of that site (flightaware) and then stick them in fpl2if to add in any fixes infinite flight might be missing (but again, do look over the charts on say skyvector for altitude and other restrictions)
Sorry about the wall of text, but flight plans are such a broad topic!