All transactions on your site are stored under Admin > Commerce > Transactions.
You may apply various filters to find particular transactions, such as within a certain date range or by category etc:
Once you've applied your filters, you can export the list of transactions via the Export button near the top.
The export will yield a zip file containing two CSV's, one for Transactions and another for Purchases.
The Transactions Spreadsheet gives you a separate row of details for each transaction, including totals paid & refunded, customer contact info, and which registrations and products were included on the transaction (purchases).
The Purchases Spreadsheet gives you a separate row of details for each purchase (registrations and products) within transactions you exported, including the transaction ID that the purchase was on, the cost, and the amount of service fees taken from the sale of that registration or product.
For example, a player registers for a tournament that costs $100 and also buys a t-shirt for $20. The Transaction CSV will have one row with all of the details for their $120 transaction as a whole, while the Purchases CSV will have two rows - one with details about the $100 tournament registration and another for the $20 t-shirt.