From 5:4 to 4:3

[Selur]

Ok, but why the input resolution is 720x576 ( Hybrid detect 1:1 anspect ratio) and if i change the input par to 16x15 , the output is again 720x576?

You need to differentiate between 'pixel aspect ratio' (PAR), the 'display aspect ratio' (DAR) and the 'storage aspect ratio' (SAR).
1:1 is the PAR, so is 16:15, the PAR is only signaled as a flag which tells the player how to resize upon playback.
If you change the PAR you do not change the 'storage aspect ratio'.

there is a resize to the same size?

i can add the PAR values on a 2nd time after rendering? (are these values just something hardcoded on the CLIP to make players able to understand what "kind" of pixels geometry is used?)

PAR signaling can be stored on the stream and the container level.
The container level is usually easy to change.
The stream level can only be changed on some formats using ffmpeg bitstream filters (https://ffmpeg.org/ffmpeg-bitstream-filters.html), on others a reencode is needed.

Usually the DAR is not signaled, it's calculated using PAR and SAR, which is why Hybrid only changes the PAR and the SAR.
Since DAR is calculated, you change it by changing PAR and or SAR.
Since changing the PAR does not change the SAR, it also does not change when you crop or resize (those change the SAR).
Options under 'Pixel Aspect Ratio (PAR)' change the PAR.
'Picture Crop', 'Picture Resize' and 'Letterbox' change the SAR.

Cu Selur