First, to get pedantry out of the way, Baltimore is a city in the state of Maryland. The Oakland Raiders are moving to Vegas in the off-season, so they will have a team soon.
Lastly, I wouldn’t say Mahomes came out of nowhere. They managed him well, but he was taken with the 10th pick in the 2017 draft as the 2nd quarterback (only one additional quarterback was taken until late into the third round - the other two being Mitchell Trubisky and Deshaun Watson). In college, he set single game NCAA records for most offensive yards in a game and most passing yards in a game, and led the country in total offensive yards, passing yards and touchdowns in his final season.
The only reason he didn’t get rookie hype was that the Chiefs smartly waited for him to learn the offense in his rookie season as they had a very capable veteran QB in Alex Smith for him to learn from. As soon as the season was over, they let Smith leave for the Redskins because Mahomes was always their plan for the future.