I haven’t been involved in an ROTC program for 12 years, but, to the best of my knowledge, not much has changed.
First, as JZStrope stated, ROTC is a for year commissioning program for one of the three Departments of the DoD (Marine Corps Commissioning falls under Navy ROTC). My experience is entirely in Army ROTC.
Second, you can participate in the first year of classes without committing to join the Army after college (note: this may be two years, google ROTC for the latest information)
If you do not have a ROTC scholarship out of high school, most programs will allow you to compete for a scholarship after you have started, I would recommend checking this on the web, and speaking with the recruiting Officer at the college/university you will attend for more details.
Third, ROTC commissions officers into both Active component and Reserve component. This depends on a number of factors, and it’s been too long since I’ve been involved in the program to know what those factors are. Again, if you are interested, do your research, and speak to the staff at your university.
Fourth, PT will affect your ability to gain mass, to an extent. If you join an ROTC program, bodybuilding should become the 300 meter target, first focus on maxing the APFT, rucking with full kit, and developing strength in your grip and posterior chain. When your battle buddy is bleeding out on the side of the road, know one cares how big your biceps are, unless you can treat the wounds and move him/her to safety. You can still develop plenty of strength. Last year in Kuwait, I could max the APFT, deadlift 480, bench 300, and do pull-ups with an extra 100 lbs, at 175. Granted these numbers aren’t great by powerlifting standards, but still pretty decent. Developing the strength required to max the APFT will provide a good base for your strength and physique goals.
Fifth, and final point, my time on active duty was extremely rewarding. I meet my wife there, made life-long friends, and experienced amazing things. I also lost good friends, saw a lot of human suffering, and made decisions that I question to this day. ROTC is the beginning of a path where you may one day be expected to lead young men and women into combat, make sure you understand that this is not just a college course.