Quick home electric question

That looks like a power meter base.... where a power meter would be installed. It completes the connection from the electric company's power meter feed to you panel... but the input power connection from the pole is missing. That would be feeding into the top connections that you show.

So it sounds like the 'garage panel' is feeding the whole house. But a 100A feed is 40-50 years behind the times for a house feed.... a 200A feed has been the standard since the 80's or earlier. I can only guess what is going on... it is almost like the installation of a new panel started and was not finished. Maybe the prior owner did not pull a permit and so was doing it under the radar and got scared and stopped....??? Who knows?

Now to your generator connection.... please forgive me for saying the following, but won't pull any punches here as it involves life safety. What you are planning to do is dangerous. If you have the generator feeding into your panel and forget to cut off the main 100A input breaker guess what..... you have energized the main line from your generator. I can and does happen. If you try to feed other houses then good luck!

What is dangerous is when the power company workers come and isolate sections of the line, and they touch your line thinking it is dead; they can be electrocuted as there is another source of power on the line that they thought they had isolated. Don't doubt this... this kind of death has happened more than once. And it happens just as described... the user forgets to cut their main panel breaker and inadvertently energizes the main incoming power company line.

So if this is what you are doing, please back up and get a proper whole house transfer switch, which, when you throw the switch, properly cuts off the connection to the power company feed before it makes the connection to the generator. And knowing where you live, to be legal you may need an electrician to do this. You at least need to pull a permit and have it inspected.

Best course for now IMHO... run some extension cords from the generator when you need it, and run the minimal stuff you need directly.

Sorry to be negative, but I am honestly concerned about this.