Change input.mp4 to your input file (can be different extension) and output to the whatever output name you want. Won't increase video quality, just useful to upscale 1080p to 4k quickly so YouTube encodes your video better.
ffmpeg -hwaccel cuda -i input.mp4 -vf "scale=3840:2160" -c:v h264_nvenc -preset p3 -rc vbr -cq 19 -b:v 0 -c:a aac -b:a 192k -movflags +faststart output.mp4
Phones often record audio in a way that Davinci Resolve disagrees with. This conversion command fixes that for you. It's a for loop that converts all .mp4 files in your folder to a PCM_s16 .mov that Resolve reads properly while maintaining data integrity.
for f in *.mp4; do ffmpeg -i "$f" -c:v copy -c:a pcm_s16le "${f%.mp4}_resolve.mov"; done
Not reccomended as this can possibly make audio quality worse, but some tools only work with FLAC files and this will batch convert all .mp3 files in a folder to 16 bit, 48000Hz FLAC files (48000Hz instead of 44100Hz for quicker reading speeds on modern devices)
find . -name "*.mp3" -print0 | while IFS= read -r -d '' f; do ffmpeg -i "$f" -ar 48000 -sample_fmt s16 "${f%.mp3}.flac"; done