You need to go to the California DMV and purchase a "One Trip Permit" (Reg. 402T). You can purchase a blank permit before you buy the car. After you have physical possession of the car, fill out the form and tape it to your rear window.

When I went, the clerk was not familiar with Reg 402T, but a supervisor was able to sell me one after I showed her the webpage below: https://www.dmv.ca.gov/portal/handbook/vehicle-industry-registration-procedures-manual-2/permits-and-decals/one-trip-permit-reg-402t/

From this Reddit link from 2 years ago.
https://www.reddit.com/r/askcarsales/comments/zbfjwx/buying_a_car_in_ca_when_you_live_out_of_state_can/

It isn't clear if this gets you out of paying CA sales tax.