Alternatively, I could explore hard money lenders.

You have one STR, and you want someone to bring all of the money for your second?

U need to flip the return  75% to the money and 25% to you if have any hope of finding someone to do this that is not friends and family.. 10 to 15% is way to little for the risk of being a gap funder.. if the project does not work gap funder is the one who loses money..

Run by Dutch Mendenhall and Amy Vaughn, they solicit hard money loans (which they do not pay back) and joint venture real estate deals which they also do not pay back. 

He has been saving his money for years and has plenty of money saved but no experience with real estate investing.

Keep saving money. 

I also have some capital to contribute.Could someone explain how syndications are typically structured, and how are people using Other People's Money (OPM) to build large portfolios with little to no money down?

Or US govt giving money to Iran.  

You didn't mention having other money available.

I never even would've thought of that.I think you need help with just money management.