Most candidates research a company the night before the interview, if at all. Doing it before you apply is more useful, because it changes what you write and saves you from pursuing roles you would decline anyway.
Start with what they sell and who pays
Before anything else, understand the business model. What is the product, who buys it, and how does the company make money. Surprisingly many applicants cannot answer this at interview, and it is immediately obvious.
For a public company, the annual report answers this in the first few pages. For a private one, the homepage and pricing page usually suffice.
Work out where your role sits
Is the function you are joining a revenue centre or a cost centre. Teams that generate revenue tend to have budget, influence and hiring momentum. Teams treated as overhead tend to be the first cut in a downturn.
This is not a reason to avoid the latter, but it tells you what to ask about job security and investment in the team.
Check whether the company is growing
Look at their careers page. A company hiring across many functions is expanding. A company hiring only replacements for departures may be flat. A company with many long-open roles in the same team may have a retention problem.
News coverage, funding announcements and layoff reports fill in the rest of the picture quickly.
Read employee reviews carefully, not literally
Review sites skew negative because people write when they are unhappy. Ignore individual complaints and look for patterns. If ten reviews across three years mention the same management problem, believe it. If one review is furious about a specific manager, note it and move on.
Pay particular attention to reviews from people in the function you would join. A great engineering culture tells you little about the sales floor.
Look at the people, not just the company
Find the team you would join. How long have they been there. Where did they come from. Rapid turnover in a small team is a warning. A team of people who joined from companies you respect is a good sign.
Also look up your prospective manager. Their background usually tells you more about your day-to-day experience than anything on the company website.
Read the job posting as evidence
A well-written posting with a clear scope suggests a team that knows what it needs. A posting that lists the responsibilities of three different jobs suggests either a stretched team or an undefined role. Both are worth knowing before you invest in applying.
Find one thing to reference
The practical output of all this is a single specific thing you can mention in your application and interview. A recent product launch, a stated strategic priority, a market they just entered.
One accurate, specific reference does more than a paragraph of general praise, because it proves you did something almost nobody else did.
Decide whether you actually want it
Research is not only for impressing the employer. It is also how you avoid spending six weeks pursuing a role you would be unhappy in. If what you learn makes you hesitate, that is a useful result, not a wasted afternoon.