There's so much to dig into on this. And it depends on the company. For SAAS, you need to have a trustworthy developer that has worked on similar types of software systems in the past and do due diligence with that person trying to evaluate the quality of the code, the architecture of the system, the team's capabilities (if possible), and improvements they think are necessary. Ask about the processes in developing any new features or fixing bugs and ensure you understand the organization.

For General IT - like computers and internet in an office, it should be much simpler - and less likely to be as big of a risk.

For Ops systems - like software the company uses to operate, you should review how the systems work and integrate with the day to day operations of the company.

This is very general advice based on the generalness of the question.

Yes, exactly