While Google started automating its build tests in 2003, the engineering industry took longer to do the same. But automation was sorely needed: Software systems are growing larger and ever more complex… To make matters worse, new versions are pushed to users frequently, sometimes multiple times each...
Jenkins is not a modern CI. It is a historical milestone, but if you read an article you should see that it was replaced by other tools. Now I don't recommend considering Jenkins for new projects. It it fast to set up but extremely hard to support and full of old bugs and legacy code. Writing Groovy pipelines is much harder than pipelines in gitlab/github/forgejo/etc. Tens of plugins you have to use even for simple tasks have inconsistent syntax, many of them are buggy, they often become unsupported or deprecated. This all consumes lot of resourses: I maintain an instance that eats ~4G of RAM being idle.
The feeling of reading through those crazy JVM stack traces with classes named "hudson" from the Jenikins prototype... I shudder! Well done for pushing through it all!