My best practice is that you should put all projects in one GIT repository, which will be released together as you put and share the whole repository in GIT. If the project has a different release cycle, put it in a different repository.
Example:
You have 5 projects A, B, C, D and E. A, B and C for sample projects that represent one level in the zour architecture of your application. D and E are helper libraries that will change sometimes, but not with every version of your application. (e.g. additional GUI elements, ...)
In this case, I would use three repositories:
First repo:
Parentpom for A, B, C
A as module of ParentPom
B
C
Second repo:
D
Third repo:
E
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Jenkins Job , , . , Jenkins Jobs ( pom )