You tell Sonarr what shows (seasons, episodes, or complete) you want to watch. For episodes that aren’t out yet (ie something that airs tomorrow, or in a week, or next season) so long as Sonarr is watching it, it will download once available.
One an episode is released/availible, Sonarr talks to indexers (Usenet or torrent; through jackett, prowlarr, or other) to find those shows in the quality profile and language you asked for.
Sonarr then takes that torrent/Usenet and gives it to a downloader.
Once the download is finished, Sonarr moves a copy to the media folder you designate.