I know that in a project many contractors will come together in a bidding and then the one with the lowest quotation eventually wins... But is it always the case? Does the client have an influence in the choice of contractor? And I can't seem to differentiate between a consultant and a contractor.

A contractor can also be chosen from having worked with a client in the past and having proven to be reliable and efficient. So past good history. A consultant merely guides, advises or recommends a course of action or process for a client, while contractors do the actual work.

There are contractors and subcontractors. In bidding, it is not always the lowest, depending on their experience, prior work with the person making the choice.

There are 100's of factors, price is often not the most important one.