Hi, I do not understand as VSTAB and transcoding work.

When I have a clip and apply vstab or transcode them, kdenlive starts a job as when it creates proxies.

If I want to drag the stabilized files into the timeline for further editing, which files do I have to drag, the original clips or the one created after the job (sh.mlt?)?

After running the job, if I drag the files in the timeline, do I already preview the effects of stabilization or they show only after rendering?

If I have understood correctly vstab and transcode crop the image; is this correct?

Which is the ideal workflow, first vstab or transcode AND render to a lossless format and then import the stabilized files into the project or just run vstab or transcode and work with the clips straight away?

drag the one created after the job.
Yes it will crop your image. It works kinda like zooming in on the image and panning around on the shakey footage, following the shake around, to generate a stable bit of footage.
I believe the clips you end up with are a preview of the deshake. I stabilze and render to lossless and then further edit that stabilize clip.