[gst]

On my Pixel 3: Settings / Apps & Notifications / Advanced / Special app access. From there you can whitelist, e.g., Chrome to allow the installation of 3rd party apps.

IIRC Chrome even asks you if you want to enable that setting once you download an APK file.

[baroffoos]

I didn't have chrome on my phone and the default android browser doesn't request permissions. I used to install it from the downloads app thing but that doesn't seem to work anymore because of these new changes.