To see all variable fonts, check the Show only variable fonts
checkbox.

Either scroll down (or use the search field) to find Roboto.
This lists several Roboto variable fonts.

Select Roboto Serif to open up its details page.

On the details page, select the Type tester tab.
For the Roboto Serif font,
the Variable axes column looks like the following:

In real time, move the slider on any of the axes to
see how it affects the font. When programming a variable font,
use the FontVariation class to modify the font’s design axes.
The FontVariation class conforms to the
OpenType font variables spec.

Static fonts

Google Fonts also contains static fonts. As with variable fonts,
you need to know how the font is designed to know what options
are available to you.
Once again, the Google Fonts site can help.

Using the Google Fonts site