Does shotcut make the importer audio a little louder?

I create my music in Reaper and it tells me this

then I import my music into shortcut and add other things without audio and export and finally upload it to youtube however even though my first audio in reaper was LUFSI -14.1 the youtube seems to see it higher and brings the volume a little down automatically .

If you right-click in the YouTube player and turn on Stats For Nerds, it will show how many LUFS that YouTube is measuring, and how much it is turning the volume down. Combining that with the LUFS meter in Shotcut, it should be possible to tell where any discrepancy is happening and exactly how much. Just remember to hit the Reset button in the Shotcut LUFS meter before playing back the whole timeline if you want accurate integrated LUFS.

That, or bring the audio from the exported video into Reaper and check the LUFS-I there to see if it remained the same. There are variations of LUFS. Some are ungated, some are gated (count or don’t count silence). Those variations can account for different LUFS numbers between products.

And one last but unlikely possibility… is it possible that Reaper and the web browser have different per-application volume level settings in the Windows Volume Mixer?