I would first reject the premise of "unification" or "reunification" (as there was no prior unity to speak of), and recommend the more accurate term "annexation".

As to what would happen during the annexation. History is overflowing with examples of large countries annexing small countries. We are witnessing one right now in the Russian invasion of Ukraine.

Among other things, the Chinese authority might attempt the following:

• Slaughter Taiwan's political leadership. That includes the president, premier (prime minister), cabinet ministers, members of parliament, local mayors, local councillors, etc.

• Suspend rule of law. The country would be put under strict military occupation. The constitution and statute laws would be suspended. There will be no judiciary to adjudicate disputes. Power is might.

• Slaughter all dissenter. Once they're done with the political leaders, the Chinese military would seek to eliminate all dissenters. Civillians would be ecouraged to report on each other, possibly in exchange for reward. Expect seeing mass graves.

• Erase all cultural signifier of former regime. All public signage and textbooks would be switched from traditional Chinese to simplified Chinese. Taiwanese history would be replaced with Chinese history in school and textbooks. Everyone must hand over their Taiwanese passport and be issued Chinese passport.

• Deportation of Taiwanese children. Taiwanese children might be deported to mainland and be reassigned to Chinese families, in an effort to erase all existence of Taiwanese population. This has been done to Indigenous peoples in Canada and North America, and is being done right now to Ukranian people in the eastern part of the country.

... what would happen to Taiwanese people, their rights, and their legal status within unified China?

Or perhaps is there an either official or 'commonly understood' standing offer such as "If you don't resist re-unification you will get to keep these rights and this status..."?

What makes you think anyone has rights in China? Provided that there is no rule of law to enshrine those rights in the first place?

I would first reject the premise of "unification" or "reunification" (as there was no prior unity to speak of), and recommend the more accurate term "annexation".

As to what would happen during the annexation. History is overflowing with examples of large countries annexing small countries. We are witnessing one right now in the Russian invasion of Ukraine.

Among other things, the Chinese authority might attempt the following:

• Slaughter Taiwan's political leadership. That includes the president, premier (prime minister), cabinet ministers, members of parliament, local mayors, local councillors, etc.

• Suspend rule of law. The country would be put under strict military occupation. The constitution and statute laws would be suspended. There will be no judiciary to adjudicate disputes. Power is might.

• Slaughter all dissenter. Once they're done with the political leaders, the Chinese military would seek to eliminate all dissenters. Civillians would be ecouraged to report on each other, possibly in exchange for reward. Expect seeing mass graves.

• Erase all cultural signifier of former regime. All public signage and textbooks would be switched from traditional Chinese to simplified Chinese. Taiwanese history would be replaced with Chinese history in school and textbooks. Everyone must hand over their Taiwanese passport and be issued Chinese passport.

• Deportation of Taiwanese children. Taiwanese children might be deported to mainland and be reassigned to Chinese families, in an effort to erase all existence of Taiwanese population. This has been done to Indigenous peoples in Canada and North America, to Ukranian people in the eastern part of the country, and to Uyghurs in Xinjiang.

... what would happen to Taiwanese people, their rights, and their legal status within unified China?

Or perhaps is there an either official or 'commonly understood' standing offer such as "If you don't resist re-unification you will get to keep these rights and this status..."?

What makes you think anyone has rights in China? Provided that there is no rule of law to enshrine those rights in the first place?