Currently our world only these two method is commonly use it. In third method you download bootstrap CSS or sass file locally from GitHub and then use it inside your project.

https://github.com/twbs/bootstrap

If you add sass in next.js you read my this article on next.js https://medium.com/officialrajdeepsingh/how-to-add-sass-scss-in-next-js-77a2b34f1ff3