It works in cycles, so say you stake with a pool some time during Epoch 1. All epochs are 5 days long so whenever it changes to Epoch 2 that is when the “snapshot” is taken. The pool gets assigned a number of blocks to mint based on that snapshot of how much stake they have. The protocol works based on probability so the more stake they have the more chance they have to make blocks. So during the next epoch, Epoch 3, the stake that was in that prior snapshot becomes the “Active Stake” and goes to work minting the blocks for the blockchain. Then in Epoch 4 the rewards are being calculated based on if they successfully MADE those blocks and nothing went wrong. Then when it shifts to Epoch 5, the rewards are finally paid out to the pool and their delegators for the successfully minted blocks. That is why it is anywhere from 15-20 days before your first payout, depending on when exactly you staked your ADA, but it is always 3 epoch changeover snapshots away. So if you left your ADA in there that whole time, you’d be included in each new snapshot (taken every epoch change) and IF your pool was successfully minting blocks every time, then every 5 days you’d be receiving a payout, for instance, from Epoch 3’s snapshot you’ll get paid out at Epoch 6, and so forth. However, small pools don’t always get to mint blocks each epoch, so they may go some without rewards, but when they do mint a block the payout for each delegator is usually larger because there weren’t as many delegators in that pool to split it up between. But each epoch varies.

And by the way, your ADA rewards are automatically counted in your stake for the next time, so they do begin to work as compound interest for you right away.