Jenkins
Implemented by Java ，pipeline defined by Jenkinsfile, can achieve CI as Code.
Master-Slave architecture.
There are abundant plug-ins, but many of them are in a state of no maintenance, are usable but have a lot of Warnings.
The UI is a bit outdated, but the new version is barely evolving into modern society.
Drone
Can easily integrate with github/gitea/bitbucket/gogs, the pipeline is defined by a yaml file, similar to github acitions.
Gitea
Opensource Github Alternative, Can run github actions.
Gogs
Same As Gitea.
Github Actions
Easy to use, github provide a 2C compute resource to run your actions, you don’t need to maintain a linux server.