How can I apply an attribute to all companies, deals, or people?

There are a limited number of bulk actions that you can perform from the list view pages, depending on which you're looking at:

People: Add task, reassign ownership, set status, add or remove tags, set a custom field value, delete, add to a focused list, or create a deal. If you have the MailChimp integration, you can export to MailChimp, and if you are on our Grow plan you can send email campaigns.

Deals: Add to a focused list, send email campaigns (on Grow), add a task, set pipeline/stage, set status, set performance lane, set custom field values, reassign, archive, unarchive, or delete.

Companies: Add task, reassign ownership (if Executive), delete, set custom field values, and add to a focused list.