How is Zakat (Zakat on benefits/gains) calculated on varying monthly salaries, given that a portion of it is spent, the rest is saved in a bank, and sometimes a part of the money is loaned out?

If the profits are from a usurious bank, then they are forbidden riba (usury) that must be disposed of for the benefit of Muslims or the poor. The principal amount should be Zakat-eligible if it reaches the nisab. Sincere repentance is obligatory, and the money must be withdrawn from the usurious bank.

If the bank is Islamic, the profits gained are of two types: First: Growth of the principal, like trade goods, which is subject to Zakat along with its principal. Second: Not growth of the principal. The majority of scholars mandate Zakat on all acquired wealth upon the completion of its hawl (lunar year), independently. The opinion of Abu Hanifa is that acquired wealth is subject to Zakat along with the Zakat of the principal, absolutely. This is easier to calculate, but it is an advancement of Zakat before its due time, which is permissible.

As for money loaned to people, its Zakat is obligatory. The owner has the choice between paying it along with the Zakat of his other wealth or upon receiving the debt for all past years, which is the more preferable view.