Furnace Pipe What's It Purpose

Combustion intake. Described on your report as “atmosphere combustion”.
And hopefully you called out the corrosion/rust - consistent with failed condensate drainage.
This is kind of basic, so study this stuff. You’ll be better, more comfortable, and millions of innocent lives will be saved. :+1:

Wow, I should have known. Since there were no louvers in the cover, I should have assumed that pipe was for combustion. Usually they get the combustion air from the outside right? That was the first time I saw a pipe connected to a (high efficiency?) furnace like this.

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No. Not always. If the roof/basement is large enough, drawing combustion air is OK.
Older leaky homes especially.
Definitely get some training on furnaces, maybe do a ride along w/a heating contractor. Read above lots of good advice.