Bitcoin fees are normally evaluated as a rate per virtual byte, not as a percentage of value sent. Users bid for scarce block space; miners generally select packages that maximize fee revenue within block constraints.
When demand is low, inexpensive transactions may confirm quickly. During congestion, higher-fee transactions move ahead. Wallet estimates are forecasts based on recent mempool and block behavior, not guarantees.
A transaction spending many inputs can cost more than one moving a much larger value with a single input. SegWit introduced weight and virtual size, discounting witness data in block accounting.
For the clearest picture, read this entry together with Transaction, Mempool, Mining, Block Subsidy, Segregated Witness. The reverse links also lead from Mining, Transaction, Block Subsidy, Mempool.
Paid from inputs minus outputs
Usually quoted in sat/vB
Market-priced under congestion
Depends on data size, not value sent