On of the developers of SymbiFlow gave a high level update at OrConf 2018 (https://orconf.org) recently. I believe it was recorded but haven't seen any info of when they will be published. You can find the slides at https://j.mp/orconf-symbiflow but it is a little cryptic without narration.

The guy working on the ECP5 stuff also gave an in-depth dive into how that part works but I can't find a link to his slides.