Thanks for the replies, everyone.

So, just to recapitulate...

When soloing over a riff which is based on a certain scale, one can obviously solo using that same scale (or mode) on which the riff is based. Or else, there are these relative/parallel modes where one can use different modes to solo over a riff in order to add more 'flavor' and make it sound more interesting.
This is what I understand by the concept of relative/parallel modes. I don't know whether I'm mistaken or not.

Could anyone confirm whether the above is correct or not, please?