Default fields work in the same way as any other field. The only difference is that they are displayed when creating a new event/organization/contact/participant along with the Essential system fields (e.g. Name, Start & End date, Timezone for Events).
You can update a field to be a "Default field" in the Global fields manager / fields manager of an event.
You can also make these default fields mandatory during the event/organization/contact/participant creation process by activating the "Required for planners" option.