72 mln is for 6 months, so about 150 months euro (similar to last year) as opposed to Gen Digital 4BN,
McAfee not public (LLC) but estimated to be sitting at around 2 BN, Trend Micro 1.something billion (of which not a lot is through the home software division so they may be the next one to follow Norton, McAfee and F-Secure).
Bitdefender and Eset are at around 500 million euro revenue (according to web stats, they are both limited liability so they don’t report official details).
I am not developing antivirus software myself so they won’t tell me how the SDKs are managed, but there was documentation which Sophos “forgot” to secure up and it was the SAVDI engine developer guide. It has now been secured.
Developers can control a lot of parameters (they can decide what aggressiveness levels they want, do they want cloud lookups or not, do they want heuristics, file emulation). They also tell the engine which file(s) to scan, to remediate (in case they don’t have their own remediation tools) and so on.
The engine is in constant communication more often than not, through C++ COM interfaces, which will be registered at the time of installing the software (most probably an Avira official MSI will be included to do the installation and registration).