On low level fuzzilli communicates with v8 through Swift C API library in Sources/libreprl/libreprl.c
reprl_spawn_child fucntions spawns child process. It does that by creating pipes, forking itself, then setting filedescriptors, and then transforming itself using execve into v8 process. Afterwords it checks for receiving 4 byte string and it sends the exact same string back.
fetch_output fetches the output from the child and returns its size and pointer to data.
execute script writes exec, and size of script, into the command write pipe and sends script through data write pipe
Coverage information are being monitored through shared memory. On the side of v8 it is monitored through SanitizerCoverage module of Clang compiler ( https://clang.llvm.org/docs/SanitizerCoverage.html ) Through shared memory information about edges are shared with fuzzilli which implements counter for error and covered branches of the V8 code in Sources/libcoverage/coverage.c