How to customize a node--type.html.twig template?
Here is some examples 

Render the whole field markup

{{ content.field_text }}


Get just the value of the field

{{ content.field_text.value }}


Get a field value from authors object

{{node.uid.entity.field_gender.value}}


For field above you can get the machine name of the field as you see it for example in admin/structure/types/manage/page/fields

For multivalue fields look at some examples here 

Read more about how to find more variables here