Quality links from high authority domains, Relevant niche links, and the uniqueness of content are the main things for tier 1 links.
for tier 1 links manual link building is important thing. When I do link building, I like to get back links from trusted sources link guest posts, high authority sites and High Quality PBN'S... etc. They tend to give off more link power and have a better effect on increasing rankings in Google.
tier 2 links that passes some link juice to tier 1 links. Too many spam tier 2 links will eventually turn the tier 1 link into spam and So don't do this too much , If you use too much spam links and your site will get penalized. for tier 2 you can use both manual and automated tools for building link. you can use social blog posts, directory submissions, web 2.0 links for tier 2. Tier 3 links GSA and RankerX are the best choice from my opinion.