Usually the results take a little time, I believe 30 days is a good ballpark to try to see how much a particular backlink may have helped you.
It depends who is linking to you. A massive site with high authority? That’ll carry some serious juice and could be worth hundreds or even THOUSANDS of dollars for your business had you bought the link. A bunch of backlinks of garbage / throwaway sites that don’t have much authority, aren’t going to do that much for you. They’ll help, but not a ton.
Yes, links alone are backlinks, but ideally you want to make sure they’re followed links. However, the guest post strategy isn’t bad as long as you’re not sending the same post to each place. Probably a no-no. It’ll look much more organic and less spammy. “It depends” is what I’ve come to learn about SEO lmao