General History/Mexican Colonialism in Alta California
What was the degree of Spanish Colonization in the north-eastern area of the holding of Alta California?
Did the Spanish/Mexican population ever make their way across the borders into the US territories in any significant numbers?
What was the situation with those Northern Mexicans after that territory was ceded?
Spanish had almost no settlements in Alto California. The few that they had were along the west coast. I don't think there was much of anything in the north eastern area. The Spanish and later Mexican never made any serious attempts to claim land that was part of the US. They had too much land and too few people to control it already. They were far more afraid (correctly) of US encroachment on Spanish lands.
Arguably, the Mexican American War started when Mexican troops attacked US Territory in Texas. But that territory was disputed, with Mexicans claiming it was still part of Mexico. A few thousand Mexicans joined the California Gold rush in the late 1840's after California was ceded to the US. But given the number of people who streamed in from all all over the world, these numbers were not particularly significant.
Mexicans who continued to live on their land after it was ceded from Mexico to the US came under US jurisdiction. Generally, they were permitted to keep their land and become US citizens. However, a good many land owners often lost disputes over their land with US claims held by others.
I hope this helps!