.item
and .properties
files are just meadata: jobs, process, connections, repository elements and so on...
Metadata is mandatory for editing (designing) the job and for the very core structure of Talend workspace itself, but it's not needed fir job execution. This mean you can forget about it after you have exported the job.
.java
files are the auto-generated source code for the job. You cannot use it directly because: i. many of them are generated for a even simple jobs (external routines, dependencies...) ii. components generate an intermediate set of .java
files using a javajet-based preprocessor.
But you can bundle the source code when you export a job. THAT java code is self-contained and useful for intergration purpose or other advanced scenarios.