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For a full list of things you can change, visit the Minecraft Wiki on Data component format.
A non-exhaustive list of things you add, remove or change on any item:
In essence, any item, modded or vanilla, can be changed in any way you might want.\
Components on (an) item(s) can be changed by adding an entry under <namespace>:item_components/<path>.json.
Entries have 3 fields, all optional:
parents: an ID or an array of IDs
targets: an item ID, an item tag suffixed with '#' or an array of item IDs
components: a component changes object
Visit the source code for example datapacks.

Change the default components for items! Server-side and data-driven, refreshes on /reload! (and does it properly)