docs: How Renovate Works overview - Basic

docs/usage/key-concepts/.pages

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 nav:
+  - 'How it works ?': 'how-it-works.md'
   - 'Presets': 'presets.md'
   - 'Dependency Dashboard': 'dashboard.md'
   - 'Pull Requests': 'pull-requests.md'

docs/usage/key-concepts/how-it-works.md

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+---
+title: How it works ?
+description: Learn all about Renovate's workflow
+---
+
+# Introduction
+
+Renovate works by searching for all the dependencies in your project and then look for updates for each.
+
+To be compatible with the variety of dependency-naming and versioning conventions, Renovate has defined modules for each known convention, and you can define your own too.
+
+## Modules
+
+The three modules are: [manager](../modules/manager/index.md), [datasource](../modules/datasource/index.md) and [versioning](../modules/versioning.md).
+
+They are used sequentially:
+
+1. the manager module looks for files based on their name and extract dependencies from them, each dependency has a datasource
+2. the datasource module looks for the existing versions of the dependency
+3. the versioning module search for a valid version regarding the dependency's version
+
+For example:
+
+1. the `gitlabci` manager find a dependency named `python:3.10-alpine` of datasource `docker`
+2. the `docker` datasource looks for versions and find `[python:3.9,python:3.9-alpine,python:3.10,python:3.10-alpine,python:3.11,python:3.11-alpine]`
+3. the `docker` versioning takes `python:3.11-alpine` as it is compatible with `python:3.10-alpine`
+
+```mermaid
+flowchart TD
+  FEM[[For each manager]]
+  FEM --> FEF[[For each file]]
+  FEF --> FED[[For each dependency]]
+  FED --> DS[use datasources to get update list]
+  DS --> VR[use versioning to find the next valid update]
+  VR --> PR[Collect updates and setup the needed PRs]
+```