How can I get deep insight into a company?

Practical steps: get to know them. So look at the Glassdoor. Get to know them from side A, get to know them from LinkedIn, see what they portray. Look at the staff, so look at who works there, identify how they know each other, if you can. But you can actually build lots of great charts if you've got draw.io and various things you can build a great chart showing how everyone knows each other, six degrees of separation. It's fantastic. So I would say get to know them on both sides, see what they're selling and then try and work out if it's snake oil. If it's not snake oil, then if you are lucky enough to get through the interview process, don't just be the one being asked questions, ask as many questions as you can and ask real left field questions. Ask things like, "How many people have left?" Because that is a real world question.