Specifically donations to political parties and individual politicians are banned.
If a corporation wants to run a media campaign on social issues - that's fine. But if an ad features a politician, especially leading up to election, that'd be treated as illegal political ad.
The legal way to do is through legal lobyist. Companies can pay them and certified lobyists can talk to institutions/individuals/parties. But meetings are semi-public and the public is +/- aware of what is lobying what. Lobyists can't give gifts to politicians, otherwise they risk their license.
There's a workaround though. Politicians love to establish NGOs, then corporations donate to NGOs and politicians go on speaking tour in the name of NGO to cash out. But at least that limits use of corporate money for over-the-top election campaigns.