Consent is not the only basis for processing personal data. The GDPR provides for other possible legitimate bases for data processing, for example: the fulfilment of a legal obligation or the exercise of public functions.
You have the right to revoke your consent at any time. Withdrawing consent should be as simple as giving it. Withdrawal of consent does not have retroactive effect, meaning that pre-withdrawal processing is still legal.
No. The GDPR provides for some situations in which consent must be explicit. This additional guarantee applies to the following cases: processing of special categories of data, automated decision-making and international transfers.