How Can I Improve My Website's Domain Authority and SEO Performance?

Hi, my name is Murshad, and I have six months of experience as an SEO Executive. Currently, I am working at a startup company that manufactures Power Press machines in Ludhiana. I am trying to improve the domain authority of the website, but I haven't seen notable changes despite creating many do-follow backlinks. Additionally, most back-links couldn't be indexed, and I am unsure where I am going wrong. I would be grateful if you could help me.