What are an example of app that can be built for Jira cloud and Confulence Cloud

Am new to Atlassian. Please can someone tell me

1.) what Jira cloud is used for. what are an example app that can be build on Jira. Am trying to get an insight.

2.) I guess Confulence: is used for building team collaboration apps. please correct me if am wrong. Eg Chat collaborations for teams.

3.) What are bucket cloud. what kind of exemplary app can be built on buckect cloud.

Am trying to get more insight to enable me build my app for Atlassian App contest at devpost.com

Thank you