也就是content_main.xml将包含textview，edittext，button等组件。它将包含在activity_main.xml中。

所以我们可以认为content_main.xml就像在HTML中的partial。 activity_main.xml将包含您的活动全局设计，content_main.xml将包含内容。

从它们包含什么：

activity_main：协调器布局，ViewPager等

content_main：开发人员选择元素。 textview，edittext，button等。

android developer website对他们说的是什么

activity_main:

This XML layout file is for the activity you added when you created
the project with Android Studio. Following the New Project workflow,
Android Studio presents this file with both a text view and a preview
of the screen UI. The file contains some default interface elements
from the material design library,including the app bar and a floating
action button. It also includes a separate layout file with the main
content

content_main:

This XML layout file resides in activity_my.xml (activity_main),and contains some settings and Textview(other) element.