Jekyll with a serve command as above will allow _site to be the directory server and all paths are relative to that.
/index.html in the browser is really something going to translate to a file on disc like.
/css/styles.css is going to be like
And for any non-Jekyll projects you would use a more generic web server to achieve the same server task, but without Jekyll to build the site.
In a folder which has an index.html file already (not a markdown fine)
python3 -m http.server
Or in VS Code, uses the Live Server extension.