https://store.ar15.com/product.html?cat=22&pr=354

Read the benefit list with a bronze membership.

AR15.Com is different because it's not an off the shelf cookie cutter forum software

The My Topics  Button is for threads you have created

As a free account you can subsribe to threads in Technical Boards, but not General (and maybe not HomeTown?)  if the subscribe button isn't on a thread then you cant subscribe in that board.  

You do have to subscribe manually, there is no option for automatically subscribing like there is with paid accounts.  You will also need to set the email notifications manaully for each thread you want them for under subscriptions, a default setting is for paid accounts.

I'm not certain if the search engine allows a free account to search by a find threads stated by or find post replies by under options