SujinOS You can download the Adguard app but it is not necessary, just search “dns” in Settings and it will be immediately shown:

Then open it and fill that “Private DNS provider hostname”. If you encounter any issues, such as connection problems if you accidentally mistype something, don’t panic and just switch it off/set to auto, or type it properly again 😁
SujinOS They blocks ads only on websites or whole system?
Both. So third party ads in apps and games will be blocked completely, almost every single ad in a browser etc. First party ads won’t be blocked though, so ads by Google in YouTube for example, or Meta’s ads in Instagram, but even with those you will still see about 95-99% reduction in all ads you see on a regular basis.
An example with Speedtest app:

Asking me if I want to hide ads - but there is no ad there 🤭