Can I use Visio on a Mac?

Answer is No and Yes. It is a Windows only product and there is no client version for Mac. If you want to use Visio on Mac machine, you can go for Visio online that can be used in browser

View the details here

How do I open a Visio file on a Mac?

It is a Windows based application and you can’t open the Visio file on a Mac. But, there could be many third party tools help you to open the Visio file.
One such tool is here :
Please use it in your own risk. I haven’t tried this

Can I download Visio for free?

Go to Microsoft site and check for Try for free at the bottom. See below