Unless you are being terminated for a discriminatory reason, your employer can terminate you for any reason and does not have to give you any sort of warning or notice. Unfortunately, it does not that matter whether the employer's decision lacked merit or not.
As you already note, most employment in the private sector in California is "at will", unless you are protected by the terms of a contract or a collective bargaining agreement. That means you can be terminated at any time for any, or even no reason as long as its not based upon unlawful discrimination (i.e. based on sex, race, national origin, etc.) or your having engaged in a protected activity. So, not being a "good fit" and not providing you with written or oral warnings doesn't matter. You have no options or recourse to explore, as harsh as that may seem...
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Unless you are part of a union or some government entities that provide for grievance or other forms of hearings to challenge the employer's decision, you have no recourse, no way to legally challenge the decision. You are out of luck with no options to explore further.
You note that you understand all that the at will relationship entails. You therefore probably knew you were going to get this answer anyway. I wish you the best of luck.
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An employer can an adverse employment action for any reason or no reason, except for a prohibited reason such as discrimination or harassment based upon race, religion, gender, sexual orientation, military service, disability, etc. or retaliation for opposing illegal conduct.