You may not get to know about everything your competitors are doing. Here are few ways how you can find out some basic things about what your competitors are doing
1) On page metrics like meta tags, titles meta description, structured data mark up can be seen in "view page source"
2) Number of back links: Best source is Ahref
3) Authority of back links: Best source is Ahref
4) Frequency of link ingestion: Best source is Ahref
5) Number of index pages: seo quake or type site: site name in google
6) Interlinking style: seo quake can show what all pages have links on home page
7) If some other site is redirecting its power?( you can report)

8) Age of domain: can use seo quake
9) What cms is being used (ask google, there are many sites which do that)
10) If wordpress, what wordpress theme is being used ( there are many sites which do it and you can also find it by typing "wp- content/theme" in source code)
11) Loading time of the site: again Google it
12) ***More than all check if the content on the page is interesting
There are many more things to know but these few basics what I want to look in competitor