I have set up my computer to function as a server. When it comes to registering a domain name, I'm unsure which DNS servers to specify. However, I do have a static IP address, which is 81.30.159.5 and my DNS servers are 81.30.159.4. These servers were provided by the domain registrar.
But how will the provider know which IP address to direct its queries to?

It's your provider who manages the DNS for your domain. Therefore, it's important that he/she registers your domain in both the forward and reverse zones on their servers. Additionally, you'll require a static IP address - commonly known as a "white" IP address - which the service provider can offer. Don't forget to discuss this with your provider too!

When a user or application makes a request to access your website, the DNS servers associated with your domain are responsible for translating the human-readable domain name into the numerical IP address that corresponds to your server. This process involves a series of steps to ensure that the request is routed to the appropriate destination.

By specifying the DNS servers provided by your domain registrar during the domain registration process, you are essentially informing the registrar and the global DNS infrastructure that these servers are authoritative for your domain name. The registrar updates the top-level domain (TLD) servers with this information, allowing them to direct queries for your domain to the specified DNS servers.

Once the DNS servers receive a query for your domain, they look up the corresponding IP address and return this information to the requesting client or application. In your case, with a static IP address of 81.30.159.5 and DNS servers of 81.30.159.4, the DNS lookup process ensures that any queries for your domain are resolved to the correct IP address.
The association between your domain name and the specified DNS servers is stored in the global DNS infrastructure, allowing providers worldwide to access the mapping between your domain and IP address. This ensures that when a user enters your domain name into a web browser, the DNS resolution process accurately directs the request to your server's IP address.