SonarLint is completely free. SonarQube Community Edition is free. All other SonarQube editions are commercial and require a paid license. SonarCloud is entirely free for all open source projects. You only pay if you want to analyze private repositories.

SonarLint is open source. SonarQube Community Edition is open source. SonarCloud is not an open source product but is entirely free to analyze your open source projects with access to all the features.

The Sonar Solution covers dozens of modern and classic programming languages, popular frameworks and web technologies including Infrastructure as Code. Refer to the SonarLint, SonarQube and SonarCloud product pages for complete details. 

Yes, for the most part. For each product, you'll get native integration with DevOps Platforms, built-in Sonar Quality Gates and Pull Request/branch analysis. The difference is mostly in the enterprise features where SonarQube provides reports & aggregation and a choice for a self-managed solution versus in the cloud.

SonarQube integrates with the cloud and self-managed versions of GitHub, Bitbucket, Azure DevOps and GitLab. SonarCloud integrates with the cloud version of these DevOps Platforms.

SonarLint is a free IDE extension for JetBrains, VS Code, Visual Studio and Eclipse. On its own, it catches quality issues on the fly while you're coding. When combined with SonarQube or SonarCloud, it forms a powerful, integrated Clean Code solution with shared rule settings, additional language support and detection of Security Hotspots.