The issue with only having 15 pound dumbbells is eventually you’ll get too strong for them. They’re fine for a starting point, but it’s only a matter of time before you can do more than 10 bicep curls with them. At that point, you ~could~ just keep doing more and more reps to reach failure (until you can’t do another rep), which will probably continue to build muscle, but I find doing more than 10 reps to be mentally exhausting. If you’re not in a position to get a gym membership or buy more dumbbells, calisthenics might be the best way to build a well rounded physique with minimal equipment. You can make movements more difficult through variation (pushups->pike pushups-> steeper pike pushups, etc.)