Foot odour is a problem that can be caused by multiple factors. As with other parts of the body such as the armpits, it is caused by the reaction of sweat when it encounters the bacteria on the skin (which use it as a substrate).
This problem can be accentuated by excessive sweating and is not always related to a lack of hygiene. The causes may be hormonal (mainly during puberty), stress, infections, illnesses or pharmacological treatments, among others.
Whatever the origin, you can mitigate its effects with proper foot hygiene, specific deodorant products such as powders or sprays and the use of breathable socks and footwear.
If you have any doubts or difficulties solving the issue, consult a healthcare professional.