Disclaimer(s): even turning on the feature to force apps to install onto external media won't always install them on external media. Only applications that allow itself to be installed in external media will be "pushed" into external media. Performance if some apps may also degrade.

To turn it on, go into your Chromecast settings and turn on developer mode. You do this by locating the build version/number and "clicking" many times on it.

Once you have access to developer mode enabled, look for developer options and scroll through (toward the bottom) where the option to force installation onto external media is present. Turn it on.

Edited to add: after you've done this, you can also try "pushing" the app into external media after initial installation. To do this, once the developer mode is enabled, go into your settings and go to your apps. If you get more info on a particular app, you may notice a "storage" option appears. Click it and check to see if the app can be moved into the new storage.

I don't think you can.

I've tried developer mode, multiple SD cards including high speed, multiple SD reformats. In fact, I used to do it all the time on my phone, but now it looks like some interval OS update removed the option to turn external into internal storage.

The best that you can do, is turn on the developer mode option then use Settings:Apps to move your apps to external storage. But, even though it looks like it worked in Settings, often it does not. You need to use a 3rd party file explorer to see which apps largely wound up on internal storage. Then, uninstall them and never reinstall them (unless it is worth it to you to lose the internal storage).

The good news is that you can move all of the usual streaming apps entirely to external storage, including the game streaming apps (Stadia, Game Pass, GeForceNOW). The bad news is that about half the Play Store games and utilities will always land mostly in internal storage, no matter what you do.

And there is no obvious correlation between the age of the app and whether is can move to external storage. Something in the GoogleTV OS implementation seems to confuse apps about internal vs external storage. E.g., one app gave a choice for where to store additional downloaded data but mislabeled internal as external and vice versa.

I've read through the comments in here and I'm having a similar issue. I've added USB storage to my device also, and presented it as a data drive. I've enabled developer mode and forced it to use the external storage this way, but it still refuses to do so. Every app I install ends up on the internal shared storage, you can tell by the amount of free space left, despite the app being marked as being installed on the external storage. I've even tried manually moving each app from external, to internal, and then back to external again. Even after doing this, there seems to be even LESS space available on the internal storage and tonnes on the external still. I'm considering doing a factory reset of the device to see if this makes it behave any more consistently, as the external storage was added way after the fact, and apps have now been installed/reinstalled multiple times. Before i do this, just thought I'd check in and see if anyone has any bright ideas?

EDIT: I've realised my issue is that it's moving the apps to the external storage no problem, but the app DATA just remains where it is and will not budge no matter what I do. Attempting to use the wholesale "migrate data to this storage" option in the storage settings just causes it to say "could not migrate data to USB drive" too. I think this might be about the order in which things are done. My main issue storage wise is with Kodi, which fills the internal storage in no time, but honestly, 4.4GB of storage is shocking really, once you've got a few catch-up apps on there and Stadia etc it's full then too.