b) Apple is free to act as payment provider, but can't force businesses to use it.
If they decide to do it anyway:
In case a gatekeeper does not comply with the obligations laid out in the DMA, the Commission can impose fines up to 10% of the company's total worldwide turnover, which can go up to 20% in case of repeated infringement. In case of systematic infringements, the Commission is also empowered to adopt additional remedies such as obliging a gatekeeper to sell a business or parts of it, or banning the gatekeeper from acquiring additional services related to the systemic non-compliance.
b) Apple is free to act as payment provider, but can't force businesses to use it.
If they decide to do it anyway:
https://ec.europa.eu/commission/presscorner/detail/en/QANDA_...Apples iMessage is also currently evaluated. In a total coincidence Apple also announced recently that RCS support is coming.