When do miners include low-fee-transactions? Are Low-fee-transactions low priority to be included in block?

When do miners include low-fee-transactions?

Do they have their own formula to get priority?

Or just oldest transactions are first?
They will never include low fees transactions before high fee transactions because they have an incentive to mine higher fee transactions before lower fee ones. If they mine higher fee transactions they will make more money. If you make a transaction and pay a $1 fee while the average is $50, it is likely to never be processed. The miner will only process your transaction once there's no more transactions with a fee above $1.