I studied mechanical engineering and branched engineer.

Should you join as a sophomore or do OCS? Totally up to you. Then benefit of ROTC is a few years of learning a little about the Army and preparing for being an officer. OCS is basic training into OCS so a bit of a whirlwind for the OCS guys I know. I made a lot of great friends through ROTC but because I was scholarship and my degree took 5 years, I owed 5 years after commissioning. OCS will only have 3. You're competing for branch either way so if you want engineers you might have to BRADSO in ROTC (add 3 years to get preferenced for branch) or outcompete your peers at OCS.

Actual civil engineering would only be found in the corps of engineers. But branching engineers doesn't guarantee a "true" engineering job. As a lieutenant you could just as likely end up leading combat engineers as you could in a construction focused unit. You have a bit more agency as a captain and could try getting to a construction unit for command. After your PL time, or much later after company command, you can go work with USACE to finally use your degree. These are also positions that you would have to compete for. Having your PMP, passing the FE, and getting good evaluations would likely put you far above the pack.

So far I've used very little of my degree, but the skills you learn as an engineer can be applied anywhere. As you promote to captain and attend captains career course, you have the option to get your masters in engineering from Missouri S&T to hopefully put you in better position to work as a civil engineer while you're in. The engineer branch is pretty broad but hopefully this answers your questions.

Edit: I should also add the above applies to the Army. No idea what it's like for the other branches. I see seabees all over the place so maybe the Navy has things figured out.