In this real world, your enemy is your enemy; but in the SEO world, your enemy is your best friend. You will make use of your competitors, put their sites on SEMRUSH, Ahref or Majestic and extract all their backlinks in the Excel file; then you will spend the rest of your day to see where they have made backlinks so far.
You will see other things too. Some backlinks will be on Tumblr, Reddit, and some other websites where you can make a free backlink or a paid one in the form of a guest post provided you're working on the same niche.
Create a backlink where you can if you belong to the same niche - some backlinks are toxic, check on Moz Site Explorer, if it has a spam score more than 2, don't make there.
Use your common sense, follow your competitors path how they have made the backlinks by using the websites mentioned above. You just have to put the link, and extract the data in an Excel in one click, and implement for your site.