International SEO

I recommend to read the three contents in this order :

1) Best pratices with Hreflang :

https://moz.com/learn/seo/hreflang-tag

2) How to structure your international website :

https://moz.com/blog/the-international-seo-checklist

3) Case study / hreflang implementation :

https://moz.com/blog/hreflang-behaviour-insights