Since California has 53 Congressional Seats right now but will soon have 52. Who decides which seat will be removed? Since California is a Democratic State, will they lose a Democratic seat since they're the majority of California?
The California Citizens Redistricting Commission is responsible for redrawing the districts for representatives to Congress.
Following the 2010 passage of California Proposition 20, the Voters First Act for Congress, the Commission was also assigned the responsibility of redrawing the state's U.S. congressional district boundaries following the congressional apportionment arising from the 2010 United States Census.
The Voters First Act and Voters First Act for Congress amended Article XXI section 2(d) of the California Constitution to establish a set of rank-ordered criteria that the Commission followed to create new districts:
Population Equality: Districts must comply with the U.S. Constitution's requirement of “one person, one vote”
Federal Voting Rights Act: Districts must ensure an equal opportunity for minorities to elect a candidate of their choice
Geographic Contiguity: All areas within a district must be connected to each other, except for the special case of islands
Geographic Integrity: Districts shall minimize the division of cities, counties, local neighborhoods and communities of interests to the extent possible, without violating previous criteria. A community of interest is a contiguous population which shares common social and economic interests that should be included within a single district for purposes of its effective and fair representation.
Geographic Compactness: To the extent practicable, and where this does not conflict with previous criteria, districts must not bypass nearby communities for more distant communities
Nesting: To the extent practicable, and where this does not conflict with previous criteria, each Senate district will be composed of two whole Assembly districts, Board of Equalization districts will be composed of 10 Senate districts.
In addition, incumbents, political candidates or political parties cannot be considered when drawing districts.
Note: The above was also used in my answer to a different question. Democratic gerrymander in California to remove as many Republican seats as possible? [closed]
Every state, including those that neither gain nor lose seats in the House of Representatives, is supposed to adjust their Congressional boundaries every ten years to accommodate population changes, per the US constitution. Many states see this as an opportunity for even more gerrymandering (or re-gerrymandering in the case that the state legislature switches power).
Some states have tried to make the redistricting process apolitical. California's 2010 Proposition 20 was an attempt at accomplishing that. Whether this remains apolitical remains to be seen. It is quite possible that the Republicans in California will lose two seats, enabling the Democrats to gain one. It also is quite possible it will be the other way around (that the Democrats lose two seats, enabling the Republicans to gain one). And of course it is possible that the Republicans will lose one seat, or that the Democrats will lose one seat.
The idealistic intent of redistricting has two primary goals: (1) representing the population (one person, one vote), and (2) representing the will of the people. Political parties are anything but idealistic. In many states, gerrymandering has become a very precise science thanks to data analytics. California does not appear to be one of those highly gerrymandered states.
No individual seat is going to be removed - that is, no one's going to say that say the 22nd Congressional District gets removed. Instead, ALL of the districts will have their boundaries redrawn, after which there will be 52 districts instead of 53. Some of the districts may be approximately the same as the old ones, some may be very different. It depends on population changes, filtered through the politics of the redistricting commission.
All representatives must run for reelection after the redistricting, so if any two representatives are running in the same district, the voters in that district will decide which of them to send back to the house, or whether to pick someone new.
The number of "safe" districts for each party will be determined through the redistricting process, as described in the other answers.