>But they do not do it.

yet. They are moving forward with measures. YT webpage player.js no longer fetches individual video/audio stream URLs. It fetches single bundle pre-packaged on the server. Its a POST request now with only one URL parameter changing &rn=x, where x increments with every request, and ~2000 byte binary encoded body.

It requests pre sliced segments in form of

    "itag_251_type_3_src_reslicemakeSliceInfosMediaBytes_segsrc_reslicemakeSliceInfosMediaBytes_seg_492_range_77715493-77757871_time_4920.0-4922.6_off_0_len_42379"
    "itag_251_type_3_src_reslicemakeSliceInfosMediaBytes_segsrc_reslicemakeSliceInfosMediaBytes_seg_492_range_77757872-77879973_time_4922.6-4930.0_off_42379_len_122102_end_1"
    "itag_251_type_3_src_reslicemakeSliceInfosMediaBytes_segsrc_reslicemakeSliceInfosMediaBytes_seg_493_range_77879974-77987247_time_4930.0-4936.5_off_0_len_107274"
    "itag_251_type_3_src_reslicemakeSliceInfosMediaBytes_segsrc_reslicemakeSliceInfosMediaBytes_seg_493_range_77987248-78044561_time_4936.5-4940.0_off_107274_len_57314_end_1"

and pushes those directly into MediaSource sourceBuffers