Some countries, like El Salvador, have declared BTC to be a legal tender. So, you will be able to buy anything in these countries with Bitcoin.
Most countries currently do not accept Bitcoin as a legal tender. Hence, you cannot directly purchase goods and services with BTC in these countries.
You can convert BTC back to USD, EUR, or other fiat currency and use those funds for your purchases. Some platforms offer coupons in exchange for Bitcoin, which you can then use to exchange for goods and services.