Corporate websites are often just a very minor help. You will often learn more by searching for them on Linkedin. Some of their people may have posted interesting facts and news. Sites like Glassdoor are not reliable, many posts are either disgruntled employees or shills. Google the company name and "annual report". Starbucks, for example has available their annual reports for the last 4 years, annual letter to shareholders, proxy statement and presentation. For many smaller companies there just won't be much available. At the minimum you can search the state business registration database which will give you the owner(s) name(s) and address, and date of registration, Then you can google the name and it may lead to additional information. You can also try Yelp and see if there are reviews with responses and may learn from them.

I don't think any company expects you to do detective work to learn about them, so reading up on what's publicly available and accessible is always my approach. In addition to a thorough review of their website, especially sections relevant to the job and annual report, I look at their social media to find out about current happenings and priorities. I usually peruse the LinkedIn profiles of people with whom I'll be interviewing.

You'll come to the interview with 10-20% of the information that you would know working for the company. Unless you work for a direct competitor which would increase that anywhere from 30-50% depending on the industry and how confidential and locked down things are. But even for a public F500 really even if you're a shareholder you know 50% of the story at best.

One of the things I would do is search on the job you want and how many times it was listed. Even if modified descriptions. That isn't always the easiest thing to do, but sometimes one can dig up old listing with some work. If they list and re-list the job over the last few years, I would think lots less of the job and company/employer.