What is the meaning of aiding in sin, and does it include a person asking to shower before going to his job at a usurious bank, or explaining the features of a device that someone is buying to use for unlawful purposes?

Your permission for a person involved in usury to bathe is not aiding him in usury, but rather aiding the person in a matter unrelated to usury. As for explaining the features of a device: if you strongly suspect it will be used for unlawful purposes, then it is not permissible, as it constitutes cooperation in sin and aggression, and he who points to evil is like its doer. However, if you do not know how it will be used, then there is no sin upon you. Unlawful aid is anything that assists in performing what is forbidden, because means are subject to the same rulings as ends. Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression" (Al-Ma'idah: 2). Just as the Prophet (peace be upon him) forbade the sale of weapons during times of tribulation, similarly, any sale or lease that aids in disobedience to Allah is forbidden, such as selling grapes to someone who will use them to make wine.