So, you have installed Android Studio, and is setting it up for react native, buuuut
you're new to fish and it seems like it doesn't have a .bashrc or .zshrc equivalent.
worry not, just run the following, in order:
Set the ANDROID_HOME variable, here the U will make it persist through restarts, and the x will make it be exported:
set -Ux ANDROID_HOME $HOME/Android/Sdk