What's the fastest way to build backlink to my site?
Depends on what type of website you have.
Outreach and guest posts are most common type of backlinks where you don't risk the quality.
You can ask your clients, business suppliers for linking back to your website.
You can also ask them to post reviews and testimonials on different public platforms like social media and other online communities.
Publish your own research data and visual images, videos, podcasts, infographics that others can use as a reference, this way you can get a mention from topically relevant websites.
Broken link building is another way to outreach for relevant backlink.
Build your own PBN do it yourself or hire an expert who knows the stuff.
Competitor research can give you ideas what type of properties are linking back to your competitors make use of that to find new backlink opportunities.
If you have the money building you own quality PBN is the most fruitful investment for any serious project in the long term.