I'm running small Wordpress site (about 15k visitors per month) and I want to buy a custom domain and monetize it. During the registration I'm asked for personal info, I know I have to provide it. I also want to buy WHO IS privacy protection, but I have a question - it's for one year, the domain is too. What happens if I won't pay for my domain after one year - will all my personal information be visible, or will it dissapear cause the domain is no longer active? Thanks for your answers.
The short answer is that it depends on how you chose to protect your privacy. Terms vary by Registrar and by extension. For example, .je domains never reveal anything more than the Registrant's name regardless of the seller's policy - it is a decision made by the Registry managing the extension.
The TL;DR is "Choose the TLD and Registrar wisely".
If you purchased a third-party WHOIS protection service (note it's spelled as one word, all caps, 'WHOIS' as per RFC 3912) then the provider's terms govern that separate purchase, which is a different transaction from the domain name purchase. There are multiple WHOIS proxy/anonymity providers and you need to check the specific Terms for each to learn the answer.
One thing to watch out for with this arrangement is that you are typically registering the domain by proxy. You are not the domain owner, the proxy service owns the domain. As an example, here is GoDaddy's Terms & Conditions page which makes this clear. They own the domain, you pay the middle-man proxy to purchase and administer it. There are legal implications to this that you should talk to a lawyer about if you are serious about monetizing the domain.
Another approach which gives you more rights and control is to choose a Registrar that provides anonymity service as standard and by default. It's all one transaction, no third-party involvement or extra purchase to make. One I use and recommend is Gandi.net (no affiliation, just a customer) and they protect your data and never sell it by default. You are the Registrant and you directly own the domain, the Gandi privacy scheme is not a proxy service.