I'm not really well versed with codecs, but is it up to the devices or the providers (where you're uploading them) to handle playback or both? A couple of days ago, I tried to upload an Instagram Reel in AV1 codec, and I was struggling to preview it on my Samsung S20 FE Snapdragon version (before uploading and during preview as well). I then resorted to H.264 and it worked w/o any issues.

Playback is 100% handled by the device. The primary (and essentially only) benefit of H.264 is that almost every device in the entire world has an H.264 hardware decoder builtin to the chip, even extremely cheap devices.

AV1 hardware decoders are still rare so your device was probably resorting to software decoding, which is not ideal.

Instagram (the provider) will transcode for compatibility but likely the preview is before transcoding, the assumption being that the device that uploads the video is able to play it.