Concerning the ten lost tribes, I have a problem to understand what means Radak on Psalm 22:23. The English translation given in Sefaria is: "Or its interpretation may be: We who are in exile will declare the miracles and wonders God has wrought for us in our exile to the Ten Tribes who are in exile, in refreshment and union, and upon whom the hand of the nations has not pressed heavily as upon us." What allude to the words "un refreshment and union"? I thank you if you can help me!

It means that the Ten Tribes had relatively good conditions in exile, and were not dispersed like the tribe of Judah (most Jews) who has suffered greatly over the last 2,000 year exile. The Radak here is expressing the opinion (Sanhedrin 10, 3), the the Ten Tribes will eventually return to the Jewish people. To date, only the Ethiopian Jews from the Tribe of Dan, have returned, and many agree with the other opinion there (R. Akiva) that the Ten Tribes will not return.