Python opencv2 fourcc codecs?

Question

Python

There is a code:

import cv2

v = cv2.VideoCapture("video.mp4")
o = cv2.VideoWriter('outpy.mp4',cv2.VideoWriter_fourcc('H','E','V','C'), 24, (int(v.get(3)),int(v.get(4))))

fs = int(v.get(cv2.CAP_PROP_FRAME_COUNT))
f = 0

print(fs)

while f < fs:
  _,n = v.read()	
  o.write(n)
  f += 1

v.release()
o.release()

error:
OpenCV: FFMPEG: tag 0x43564548/'HEVC' is not found (format 'mp4 / MP4 (MPEG-4 Part 14)')'

while I took different codes from the site

and all give an error except mjpg which is not effective at all.

so I tried mp4v, h264, h265, hevc, mpeg, avc1 and everything does not work, but the
file was recorded normally every time! I checked it on this site and it is encoded with mpeg4 part 10 (h264)
although when I put its code or its aliases there was this error:

OpenCV: FFMPEG: tag 0x34363248/'H264' is not supported with codec id 27 and format 'mp4 / MP4 (MPEG-4 Part 14)'
OpenCV: FFMPEG: fallback to use tag 0x31637661/'avc1'

Failed to load OpenH264 library: openh264-1.8.0-win64.dll
        Please check environment and/or download library: https://github.com/cisco/openh264/releases

how the codec is not available, but is encoded anyway with its help?