The pieces fit together like this. The promissory note is the buyer's written promise to pay: amount, interest rate, payment amount, when payments are due, what happens if they're late. It's the debt itself. The deed of trust, or mortgage depending on which your state uses, is a separate document that attaches that debt to the building. It's what gives you the right to force a sale of the property if the note isn't paid, and it gets recorded with the county so anyone looking at title can see your lien. Recording practice and which instrument your state uses differ by state.
Amortization schedule is just the month-by-month table showing how each payment splits between interest and principal. A balloon means the payments are calculated as if the loan runs 25 or 30 years, but the whole remaining balance comes due on a specific earlier date, often year five or seven. So the buyer pays a comfortable monthly amount, then has to refinance or sell to hand you the rest.
On cost: an attorney drafting a note and security instrument for a straightforward commercial deal is commonly in the low thousands, title insurance and escrow are separate and priced off the sale amount, and recording fees are small. Some states charge transfer tax at closing, some don't.
"You're the bank" mostly means someone has to track payments, apply them correctly, confirm the buyer's insurance and taxes are current, and send annual interest statements. A third-party loan servicer does that for something like $20 to $40 a month plus a setup fee. That ledger matters if the deal ever ends up in a dispute, which is why sellers use one instead of a spreadsheet.