How can I use LibreOffice Draw to convert multiple jpg files which were scanned, to a pdf file in a short way. Last time I had to create "New page" then "Insert -> Image" which was a *** for about 50 pages.
IMHO this isn't a LO problem, but a workflow issue. The best solution depends on the specific requirements. If the requirements do vary, i would recommend trying to "modularize" the process, so variations will only affect one workflow module.
If you aren't sure which jpgs to convert (and if disk space isn't an issue), you should simply convert all of them (command line tool, e.g. imagemagick/convert).
To select the pdfs for merging, you could use a GUI tool like PDFShuffler. Just add all pdfs created in step one, and remove the ones that shouldn't be merged.
To edit the resulting single big PDF afterwards, use a tool like PDFSam that lets you insert a single PDF in the middle of another PDF file.
Use LO Impress' Photo Album feature: New Presentation -> Insert -> Media -> Photo Album. There, you can select multiple images to insert. If you're done, just export as PDF.
Maybe you should have a look at online tools for this, like https://smallpdf.com/jpg-to-pdf it's sometime easier than try to find a way using a specific software