SU 2019 download

Do we download SkechUp Pro and after 30 days it will automatically becomes Free?

No, the last and final version to work that way was 2017.
Now you get the trial then either buy it, subscribe to it or stop using it after 30 days.

Thank you Box.

If after 30 days, I want just the SU Free, Shall I use the subscribe option?

At the completion of your Trial period, you can continue to use SketchUp Free online. If you prefer to use a desktop client application over a web application, you can separately download SketchUp Make 2017 and use it for free instead.