If one established a trust as a single person (and owns some properties on their own), then got married years later. If they do not amend the trust and state that they got married, what happens?
I don't understand the question. Any number of thousands of things could happen in the scenario you described.
If this person passes away, does the spouse automatically get a percentage of their assets and estate (that the person bought when they were single)?
For starters, death isn't a matter of "if." In any event, the answer to this question depends, first and foremost, on whether the settlor of the trust has made a will and, if so, what the will says. If the settlor has not made a will, then his/her surviving spouse will be entitled to all of the deceased spouse's community property and anywhere from 1/3 to all of the deceased's separate property, depending on whether the deceased is also survived by children and/or parents. Property owned prior to the marriage would be separate property, and such property held in a trust would not be part of the estate of the deceased. However, depending on circumstances, it's possible that the marital community could have acquired an interest in such property. Many other facts may be relevant.
With all that said, why do you ask? What is your specific situation that has led you to come here and pose this question?
There is no free advice anymore. Sad
Well...for starters, in order to obtain legal advice, you first have to find a lawyer licensed in the relevant state, and I'll point out that none of the prior responses have come from California attorneys. As importantly, you need to provide clear and complete facts. You haven't done that in this thread, and you didn't do that in the thread linked in "Zigner's" response. In fact, in that prior thread, you were asked several questions that you ignored. As far as getting legal advice for free, do you work for free?