Learn English – ny difference between “Words I learnt today” and “Words I have learnt today” and “Words I have learned today”

They mean the same thing; behaviour is the British English spelling; behavior is the American spelling. The definitions say the same thing in different ways.

artiste is mainly used for professional performers:

A professional entertainer, especially a singer or dancer.

(source: Lexico)

while the word artist is applicable in many more situations, e.g. hobbyists, other types of arts or even outside the arts.

As for the difference in spelling, artiste looks more French and hence more exclusive (at least to me). So it does not come as a surprise that it's only used for a specific group of artists.