You do it. If you want your transaction to be quickly confirmed, you'll have to set a high fee, because you'll give a greater incentive to the miner. You can determine it in sat/bytes. The more inputs and outputs it has, the heavier it is and thus, the more sats you'll have to pay.

The developers cannot set the fee of your transactions. However, they can set a limit (in the source code) you can't recede. For example, you cannot pay 0 sats for a transaction, because most of the nodes will reject including anything lower than 1 sat/byte into their mempool, if I remember correctly. Note that it doesn't mean they're invalid transactions; they can be normally included into a block.