How can I adjust video quality based on users’ Internet speed?

What you're looking for is called "adaptive bitrate streaming." This technique is usually implemented using protocols like HLS (HTTP Live Streaming). It breaks the video into chunks with different quality levels, allowing the player to switch between them based on current network conditions. Instead of doing it all by yourself, consider using an existing service. For instance, I use BunnyCDN and it's worked well for me. You just upload your videos, and they provide you with an iframe or JS library to embed in your site.

At my last company, we initially managed different MP4 files for each resolution, then shifted to HLS for better flexibility. However, we encountered performance issues on lower-end devices, so we switched to CMAF which significantly improved performance and reduced bandwidth usage.

You don't need to handle uploading the various video resolutions manually. Ideally, you should have a setup that transcodes the original video into different formats and sizes automatically. You can do this on-demand, but you’ll need specific hardware like graphics cards for transcoding. It's often cheaper and easier to just store multiple sizes since HDD prices are low.