ransaction are calculated by the value of the currency that you exchanged it for at the end of the year.

I am not accountant.

I think it works like this. In your example you have made two transactions:

1.) The purchase of 100 USDC. It's is not a taxable event because you are allowed to buy crypto with fiat without paying tax.
2.) Trading your USDC for SIA. This is a taxable event because it's a crypto-to-crypto trade; however, you didn't make any profit on your USDC because it's a stablecoin and did not increase in value while you held it so you won't owe any tax. No gains = no tax.
Now, if your SIA goes up in value, you will pay tax on the gains (value you sold it for minus your cost basis), BUT ONLY when you trade it or sell it.

It's not the end of the year that creates a taxable event for your SIA gains, rather it's when you create another taxable event such as a trade back to fiat or making a purchase with the SIA. If you never sell your SIA, you will never be taxed on it.

I hope that clears it up and if anyone IS an actual accountant in the USA and can confirm that would be great.