It depends on a number of factors. Fonts for websites are typically grouped into similar styles, so that if you do not have one installed, the next will be used. It is also possible to download a font - Google provides a vast library - and if that fails, one of the defaults will be used.
The font chosen is also somewhat dependent on the browser you use, and obviously the OS. I use Android and the dark theme; on my Mac I use Firefox with the dark theme and there is little difference. This is intentional - my Mac will be displaying Helvetica, but if I used Windows the browser would choose Arial (unless I had managed to install Helvetica)
In any case, what you are seeing is one of the fairly common “font family” (they are not really a “family”, they are a standard grouping of similar fonts) “Helvetica, Arial, sans-serif”
You’d probably not see Helvetica or Arial on Android. That leaves the default, “sans-serif” which is not itself a font. It is an instruction to find a font on the system that is a sans-serif style, and in the case of Android this is most likely to be Roboto