Twitch must have saved your login details/Tokens in Keychain. Unfortunately, unless the App deleted these entries from the Keychain, iOS does not delete this information upon app uninstall automatically. That is a way for Apps to check if User is installing app for the first time or not.
Coming to Alexa, it might be totally different approach, Ability to find the devices on your network and may be with a combination of bluetooth Beacons.
Fortunately, you need to install full app to read this information. Unlike a Facebook, Twitter or Google Analytics library(Framework) can track you across other apps with the same Library or Framework.
For Second One, with iOS14 Apple prompts a Privacy Alert for Connecting to Other devices on network, You can simply turn it off.
Detecting Alexa App on the device used to be possible before, but it does not go unnoticed by Apple these days without some co-ordination between Amazon and Spotify.
Coming to Alexa, it might be totally different approach, Ability to find the devices on your network and may be with a combination of bluetooth Beacons.
Fortunately, you need to install full app to read this information. Unlike a Facebook, Twitter or Google Analytics library(Framework) can track you across other apps with the same Library or Framework.
For Second One, with iOS14 Apple prompts a Privacy Alert for Connecting to Other devices on network, You can simply turn it off.
Detecting Alexa App on the device used to be possible before, but it does not go unnoticed by Apple these days without some co-ordination between Amazon and Spotify.