How can I create an alias to point to an uploaded file?
I agree with Clive and base oh his comment, to redirect all files to another route you can use .htaccess route direction. So,
first solution:
add to following line in your .htaccess file.
RewriteRule ^sites/default/files/(.*)$ /files/$1 [L,NC]

note: you can use whatever you want instead of files, like assets etc.
PS: if your files are Media types, with pathauto module you can create a pattern to redirect them additionally you can let the user override them the what path he wants.
second solution:
install https://www.drupal.org/project/file_entity module and then with pathauto module you can create an alias for files.  like files/file-title.
third-solution: use https://www.drupal.org/project/path_file module

The Path File module allows content editors to upload files and
specify the url at which they will be available. Normally, when
uploading a file to Drupal with the same name as an existing file, it
is given a new name instead of overwriting the existing file. This can
problematic when file urls are referenced in content, menus, etc but
may need to be updated in the future. The Path File module allows you
to mitigate the impact of those changes by setting up a canonical url
alias which will always point to the most up-to-date file