No, if you are using OnlyFans in a browser they cannot detect if you are downloading or screenshotting. Only apps can detect that, and your phone browser doesn't care.
No, there is no way to download paid content from OF directly from OF without paying. That's how they make their money so they protect it very well. There is no legitimate tool or method to do that, and anybody claiming they are capable of doing that is scamming you.
That's why this forum exists - someone has to pay so they can see the content. But once they can see the content, there's no way to stop them from downloading it locally and reuploading it somewhere else... like here
If you buy the content yourself then you can download it using the tools in this thread. Once you have access to those pictures/videos, OF can't really stop you from downloading them locally.