It was essentially bribery.

Under Biden there was money to be granted (as in: via a grant, Congressionally approved) to Intel. Trump then held the grant hostage in return for government ownership of Intel shares.

There's also a threat that this deali supposedly eases around the Intel CEO that Trump said was 'too connected' to China.

It's either borderline or blatantly illegal, but there are likely no parties eligible to contest, or interested in contesting, it in court.

(Based on my memory of someone's breakdown of a few examples like this - there's a chance I'm conflating a couple of different but related things, the deal with Nvidia to allow selling of some more advanced chips to China being another)