How and does deno deals with extensions built in C/C++ for instance?
Maybe (just "maybe" for now) we will be able to support plugin imports. In that case dylibs could be directly pulled down from network just like normal files (don't take my words as official though -- we have not decided on that yet)
There have been attempts to add some util functions for loading such plugins easier: https://github.com/denoland/deno/pull/3471