In Chrome, click the 3 dots in the upper right hand, cast, select device.
Plex Server ftw
I use KODI which is connected to a file share on my pc.
Purely from a local file on a PC to Chromecast, VLC is probably the easiest.
VLC --> Playback --> Renderer. Should see your detected cast devices there.
Plex is the best option, but requires more setup.
/r/Videostream is a super easy solution
Literally just this ... you'll see once you use it, ultra simple and works perfect
If your intent is to run games off your PC via Chromecast, you're not going to be happy with the latency. However, if your Android TV device is an Nvidia Shield TV and your PC has a gamestream-compatible Nvidia graphics card, I believe you can use gamestream to steam anything over your network to your TV with little to no lag. (Don't ask me which cards are compatible. This is the extent of what I know.)
I use Kodi to read my network share, it even downloads all the art for movies and tv so it's like having your own personal netflix.
In Windows share the folder that has your media and then watch with Kodi on Android TV.
To share the folder, right click on the folder, click Properties, click Sharing, then click Share.
If we're talking TV shows and movies, then use a proper media center setup like Plex.
If this is just random video clips that you download and delete immediately after, then use the Chrome browser.
There are a few different methods. Each has its pros and cons. IMO, SMB share or UPnP server is probably a better option for consuming media files stored on PC.
https://www.howtogeek.com/233810/how-to-watch-local-video-files-on-your-chromecast/