Can I connect through LAX to PDX without going through quarantine? [duplicate]

Strictly speaking, the answer to your question is "yes", you can avoid quarantining, since (so far as I can tell) the California quarantine rules are merely advisory and do not have the force of law. From ABC 7 San Francisco:

SOUTH SAN FRANCISCO, Calif. (KGO) -- On Friday [Nov 13, 2020], Governor Newsom issued a travel advisory, asking those coming into California to quarantine for 14 days and asking residents not to travel out of state during the holidays. Unlike other countries where there is a strict mandatory quarantine, the advisory is just that -- advice, with no one tracking you down.

Travelers arriving at LAX are being asked to sign a form acknowledging that they understand this advisory. There is a possible fine of $500 for not submitting this form, but signing the form itself does not appear to bind you to any course of action. It is also not clear from reporting whether connecting passengers connecting through the airport are being asked to sign this form.

LOS ANGELES (KABC) -- Starting the day before Thanksgiving, travelers arriving to Los Angeles by airplane or train will have to sign a form acknowledging California's recommendation of a 14-day self-quarantine, Mayor Eric Garcetti announced Monday evening.

Travelers over the age of 16 arriving from another state or country must submit the form online prior to or upon arrival at Los Angeles International Airport, Van Nuys Airport or Union Station.

"To help stop the spread of COVID-19, Los Angeles is requiring all travelers entering the city from another state or country to fill out this online form, acknowledging that they have read and understood California's travel advisory," the form states on the city website.

[Whether any sort of travel is a good idea at this time is a different question, with a different answer from the question you asked.]