I think it’s safe to say that about 90% of what you see on a red carpet is borrowed. Not many celebrities want to pay for their dresses (which are often very expensive), when they have the chance to simply borrow them or, if they are A-list celebrities and participate to really important events like the Oscars, to even get paid for wearing them.
Jewellery is also mostly borrowed and sometimes an actress or celebrity will sign a contact with a brand, and wear it at different events. Depending on the importance of the evening, the exposure brands get is huge. A stylist is often part of the game and gets paid too for choosing a certain brand.
Allegedly, it was Halle Berry starting it all in 2000, when she charged 50k $ to wear Bulgari at the Academy Awards. Other celebrities like Angelina Jolie or Oprah Winfrey won’t get paid.
In the case of less famous people (like myself!), dresses are simply borrowed. Any online or media exposure is good advertising for a brand, so designers (or PRs who take care of a brand) are willing to lend some of their creations to people who have a good media exposure.