How can i know which keywords people searched to get to this sits

Hello experts, coming from development i try to learn SEO .
For example:
how can i know what people searched to get those sites :

https: // pdflayer dot com
https: // pdfshift dot i o
https: // docraptor dot com
https: // screenshotlayer dot com

as you can see they all have common ground to convert HTML to pdf.

also, can i know how much traffic each site gets?

Thanks for your help

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