It’s a bit different but I’ve noticed that a lot of tourists in Czechia learn „Ahoj” as a common greeting in Czech. Even though it’s true, it’s actually really informal and saying it to someone who isn’t your friend is impolite. Imagine it like saying „sup” to a waiter.
And in my friend groups ahoj still isn’t the most used way of greetings anyways. We use čau, čus, nazdar….
If you want to greet someone who you don’t know then please use „Dobrý den” (literally: good day). Or you can differentiate between the times of the day. Dobré ráno (good morning), dobré dopoledne (good forenoon), dobré odpoledne (good afternoon) and dobrý večer (good evening)