What’s the best way to reach out to alumni if you’re interested in working at their company?
Yes, it’s fine. Many tech companies have a referral bonus for employees that help them make a good hire, and any alumni you contact will be able to forward your details straight on to the recruiting department.
Just keep it short and to the point: “I found you in the XXX alumni directory, and I’m interested in finding out more about job openings at your company”. Include your CV and see what happens.