The deduction for state sales tax is an estimate of how much sales tax you probably paid during the year, based on your income. It's not just for major purchases. The estimate comes from a table published by the IRS. And yes, you get it automatically if it's more than your state income tax, or if you live in a state that has sales tax and no state income tax.
If you purchased certain specified items you can add the sales tax for the specified items to the amount from the IRS table, subject to some restrictions. The specified items include a motor vehicle, aircraft, boat, a home, or "substantial addition to or major renovation of a home." This applies to only the specific items listed, not to any "major purchase."