It seems the main trade (by value) is in electronic components (broadly speaking), both ways. It's somewhat easier to find Taiwan's exports.
In all, Taiwan exported $188.91 billion in goods to mainland China and Hong Kong in 2021. More than half were electronic parts, followed by optical equipment, according to Taiwan’s Ministry of Finance.
And SCMP concurs on that with data on the other side:
Integrated circuits, also known as chips, accounted for 62 per cent of the island’s exports to the mainland last year, with the value reaching US$155 billion, according to China Customs figures. And in the first half of this year, the mainland imported US$79 billion worth of chips from Taiwan.
Meanwhile, tropical and citrus fruits, along with types of fresh and frozen fish, which Beijing has targeted with trade suspensions, accounted for only 0.2 per cent of Taiwan’s export total in 2020 – a year before the mainland began imposing trade sanctions on such products.
It's somewhat harder to find a breakout of Taiwan's imports (at least if you don't speak Chinese). One Western source says
Put simply: China supplies key raw materials such as rare earths and low-end mass-produced electronic components, while Taiwan exports high-end semiconductors and optical components to the mainland to make up for China's lack of know-how.
But interestingly, SCMP seems to disagree here
Integrated circuits account for most of Taiwan’s imports from the mainland, and the island bought more than US$20 billion worth of chips last year. These comprised 28.58 per cent of its total imports from the mainland.
I suppose it's less highly integrated chips they import or something like that...