Merge audio and video in ffmpeg

ffmpeg -i D:\Audioname.ac3 -i D:\Video.mp4 -acodec copy -vcodec copy D:\VideoFull.mp4

extensions can differ

Cutting video without changing quality using ffmpeg

for example cut 5 min video from 10min  to 15 (10+5)
ffmpeg -i D:\videofile.mp4 -ss 00:10:00 -t 00:05:00 -vcodec copy -acodec copy D:\videofileSmall.mp4