How can I Use website 1 to increase SEO for website 2?

Hello everyone, quick question.

I’ve a quite established website with a decent DA score and also a freelance website that’s about 4 years old.

How can I use the SEO strength and Domain Authority of website #1 to help promote and improve the seo of website #2?

I do the odd link to it but I’m sure there’s other smart ways of helping to drive more traffic and authority.

Advice would be greatly appreciated.