You tell them you want to buy something. They find it and buy it for you. Just like buying anything else through a brokerage.

The vast majority of bonds you’ll find available are on the secondary markets, because that’s where the vast majority of bonds are.

If they offer the option, you may also be able to place bids at an auction through them.

I’ve never specifically used Merrill, so I can’t speak for them specifically how they display those options (as far as whatever yield you believe you are seeing). You may simply be mistaking which bond you are looking at. They may provide an estimated yield based on current market trends, as a guide, with no guarantee that’s what you’ll get. They may be offering to make a competitive bid at that price or better, with no guarantee you’ll win the auction. Or something else. Either read the description, or call and ask.