I will try to give you a brief definitions of the concepts in order to help you to build a better overview about the search components.
Crawling is simply a process of gathering documents from various sources/repositories, making sure they obey by various rules and sending them off for further processing to the Content Processing Component.
Read during Gathering.
Write towards Content Processing Component.
You can follow the counters of mssdmn.exe process on the task manger performance monitor.
Content Processing Component:
The content processing component processes crawled items and sends these items to the index component. The content processing component performs operations such as document parsing and property mapping. It also performs linguistics processing such as language detection and entity extraction. The component transforms crawled items into artifacts that are included in the search index. The content processing component also writes information about links and URLs to the link database.
You can follow the counters and activity of noderunner.exe (PID- Task Manager) process associated to NodeRunnerContent (PID - ULS).
The Reading is lower since it has the info already and it is mainly busy writing properties in to the indexer.
Users browsing is another different process associated to w3wp.exe
SharePoint Search is a combination of NodeRunnerQuery and NodeRunner