Are the changes public or can you add anything you want?

All changes are public, but you can add anything you want subject to the following caveats:

• You have rights to use the information added
• You don't add information about private persons (e.g., A B lives at this address) and in particular in Europe where GDPR applies.
• The information is not ephemeral or constantly changing (e.g., short-term roadworks, traffic information)
• The information is capable of being verified by someone else visiting the relevant location (the ground truth rule).
• In general its best to use widely used tags to add information about objects, even if their definition/usage seems odd

It's also worth considering any personal privacy which you may wish to maintain. A typical OSM edit profile can reveal quite a lot of information about a mapper: where they live, where they work, where they holiday, where their relatives live etc.