"They want me to give them a sign" or "Many of you want me to give you a sign." The information about what kind of sign they wanted can be made explicit. AT: "Many of you want me to perform a miracle as proof that I have come from God" (See: [[rc://en/ta/man/translate/figs-explicit]])
"what happened to Jonah" or "the miracle that God did for Jonah"
# For just as Jonah became a sign ... so too ... this generation
This means that Jesus will serve as a sign from God for the Jews of that day in exactly the same way as Jonah served as a sign from God to the people of Nineveh.