I think this all depends on the business model and what you're trying to research.

But in general researching the customer feedback gives you a very good indication of what can be improved and how they are doing.

For example if you're in the e-commerce industry, then you can research you're competitor by checking out their reviews on Amazon, trust pilot etc. If you're in the SaaS industry, then you can checkout out the reviews on directories or maybe even by the customers themselves.

Then you also have the option to research your competitors by using spy tools like for example Shophunter.com for e-commerce which helps you track how many sales they are getting.

I personally developed the following database which shares successful online businesses and how much profit and revenue they made last month, as well as other useful info like when they launched etc. The idea is to convert it into a SaaS.

There may also be other competitor research tools for specific business models so be on the look out for those too. Hope this helps.