What are the steps for a beginner to buy Bitcoin?

I understand that you need to buy cryptocurrencies through an exchange, but don’t understand how the process works.

Is it like an account that you sign up for and access via a website, then pay your standard money onto it and then trade the money you have in your exchange account for BTC?

If that’s the case, how do physical wallets play into this?

Also, my understanding of one of the benefits of crypto is that it is decentralised, but does accessing it through an exchange not ‘centralise’ it?

Thanks.