can you do some tests?

I hope you have some other machine (for example a phone).

now download the same file on both devices (ideally do it for more than 1 file, and pick something which is static)(static here means file does not change. if for example you download a video from a site, and they encode all their streams live, there is possibility that file result is slightly different).

now find hashes for both the files. i am not going to explain what is a hash here, but if you want a brief, imagine it being a unique id for each file.

to find it, go to terminal and type

md5sum <path> where path is the file path (to know the path, you can often just drag a file from your graphical file man manager into terminal and it should auto fill the path).

this will produce a strange looking string.

do this for file on both devices (if you are on a phone, this is hard. instead, you can try to copy to your linux desktop, preferably with a wired connection to reduce variables).

if both hashes are same, the files are same. in this situation, your file viewer/player is broken. there are possibilities of your memory being bad or drivers being bad but these are miniscule.

there is another possibility that your drive is either out of space, or out of inodes (something fancy, related to filesystem, not explaining that here). input/output error can often happen from such things. in this situation, best course of action is boot a live iso, and clean the disk (for example, if it is a btrfs partition, and it keeps making snapshots which are not cleaned properly, or some bad program keeps creating extremely tiny files, extremely fast).

we need a lot more information if this is the case.

if file hash is different, then it is simpler - either your downloader is borked (use a different browser, or use command line downloader, like wget/curl/aria2), or your disk is broken, or your file system is borked (something similar to above but more pronounced).

changing browser/downloader fixes the problem - you are golden. if not, try swapping out the ssd, and reformat a fresh install.