For Pixel 2/XL installation of the beta build and other devices with A and B partitions : https://forum.xda-developers.com/sho...47&postcount=4
In other words:
1. Close the XposedInstaller app
2. Download the zip
3. Copy it to /storage/emulated/0/Android/data/de.robv.android.xposed.installer/cache/downloads/framework
4. Open the XposedInstaller app
5. If the zip is not showing as available to install, click the 3 dot menu
6. Check the "Show outdated versions" check box
7. The zip should show up in the app now, and should be installable through the app
Credit to @wanam
for being the first to post this method.
TWRP installation doesn't work.