I searched online and they said it's 0.25%-0.30% per year. But since my balance is ever-changing, how do they really calculate the interest? Do they just snapshot your balance on a certain day of a year or calculate the average amount, to multiply by interest rate?
The interest is calculated on a daily basis.
There are several algorithms described in Wikipedia how exactly to do that, but roughly spoken, you accrue interest for every day with the balance for that day.
For example, if you put $1000 into your account, keep it there for 20 days, then add $2000 and withdraw $500 the day after, after another 10 days you accrue interest for ($1000 * 20 + $3000 * 1 + $2500 * 10) * p/100/360.
OTOH, if you add $10 every other day, you accrue interest for ($10 * 2 + $20 * 2 + $30 * 2 + …) * p/100/360.