How to search by tag in hierarchy?

I don't think you can do it out of the box. Check the Asset Store.

https://assetstore.unity.com/search?q=tag&k=tag

If you're up to the task, you could try to learn how to code your own Editor script that does it.

An Introduction to Editor Scripting (official guide by Unity) https://unity3d.com/learn/tutorials/topics/scripting/introduction-editor-scripting

Unity Editor Scripting - Filtering Hierarchy by Tag or Layer (nevzatarman's blog)

https://nevzatarman.com/2015/01/10/unity-editor-scripting-filtering-hierarchy-by-tag-or-layer/

For anyone reading this in the future, Unity now has a Quick Search package which does wonders in terms of search within prefabs, scenes, hierarchies, project files - you name it! And it support editing thousands of objects at the same time! Here is the cheat sheet with all filters: https://docs.unity3d.com/Packages/com.unity.quicksearch@2.1/manual/search-syntax.html