You only declare it if the broker sends it to the IRS, otherwise don't worry. The ones that send it to the IRS are Coinbase and Mercado Bitcoin.

I'm from Brazil, I bought 1000 reais in crypto this December here on Binance, I've never made a single withdrawal to my bank account, can you tell me if I'll need to declare it next year?

Note: my annual income does not exceed 20 thousand reais.