Custom fields can be added to donation forms using the Donation form in the Form Builder. Custom fields are added in the billing/payment section of the donation form.
If a custom field is added to a platform donation page, the field will appear on the platform donation page only, and not on individual campaign donation pages within that platform. Donations that are made to fundraisers on individual campaigns will capture the information in the custom field, but only if there are no custom fields on the individual campaign. The data is viewable in the "Donations - All Campaigns" report. This also applies to the Global Standalone Donation page.