Linux

Probably the most reliable way to share files is with NFS. See https://wiki.debian.org/NFSServerSetup and https://wiki.archlinux.org/title/NFS#Server_configuration for a guide.

You can quickly share files with python: python -m http.server 8000

For setting up quick download/upload access you can use shareboxx. SIngle binary web UI.

Windows

HTTP File Server (HFS) can host a directory, upload is possible as well.

WebDAV

You can share files (download/upload) using a WebDAV client/server. PicoDAV is very lightweight, requires a properly configured web server.

KaraDAV is a WebDAV server compatible with nextcloud.