I've understood that staff is one system of 5 note lines and the term for both systems (l. & r. h. keyboard) is grand staff, is this correct? ... while in a full score would be several staves?
Your understanding of staff and grand staff are correct.
And while full score is a collection of various staves, we often group these several staves of concurrent music together with the name "system." A system is just the collection of all instruments playing at a given time; in an orchestral score, a system may take up an entire page, but on a piano score, there could be several systems of music on a single page.