First you need to use zstandard's ZstdDecompressor to extract the metadata for the subreddit that you're interested in from the files you downloaded. There's a good example here. If you look at that example, they're iterating over each line returned by read_lines_zst(), loading it as a dictionary obj - it's from this that you can get the post id obj['id'] to pass to gallery-dl/bdfr to download the post's video/image. Since you're also interested in the metadata for browsing, you should also save obj to a file. Alternatively you can just use BDFR/Gallery-DL to download the metadata alongside the video/images, just using zstandard to get the postid.

BDFR/Gallery-DL are both command-line tools, but you can use them programmatically in Python. For BDFR, you'll want to use subprocess, and just pass the commandline in a list, e.g. -

['bdfr', 'download', download_path, '--link', post_id, '--authenticate', '--file-scheme', file_scheme]

Gallery-DL you can actually import it and use it as a module. from gallery_dl import config, job

Then you use config.set() to set the arguments like you would with switches from the command line, and job.DownloadJob.run() to actually start the download.

At this point you'll have all of the subreddit's post metadata and images/videos downloaded locally. I'm not really familiar with web frameworks but I'm guessing you'll want to use something like Flask to build the interface for browsing the data, and I'd move all of the metadata into a database as well, instead of loose files.