We are in Lightning App Builder, so let's go to the app page first. We can click on Edit too. So, you have the Activation… button. This is nothing but assigning the app page to the users. In PAGE SETTINGS, you can see the app name that will be visible in the Salesforce mobile app; you can make the required changes:
Then, to activate the page, click on Activate for all users or Activate for System Administrators only; you can take a decision whether you need to activate the page for the system administrators solely or for all users.
In the MOBILE navigation menu, if you want to add a page, click on Add page to app, then you add your custom TestAppPage:
So, you place this TestAppPage under the Smart Search Items, so it will be shown as an app, then come to the LIGHTNING EXPERIENCE tab, for the applications this app page has to be added. You can add Sales, Sales Console, Service Console, or your custom application. We have added this app page to Sales as well as TestLightningApp, a custom Lightning app, save it:
