For a brief overview of how software engineering is done at Google, see my paper "Software Engineering at Google" <https://arxiv.org/abs/1702.01715>, and with regards to architecture, see in particular sections 2.9, 2
11, 3.3, 3.4, and 4.1.
We typically talk more about "design" than "architecture", so for example our architecture is typically documented in "Design Docs".
It's common for code at Google to be rewritten without reusing most of the existing tests -- instead, new tests are written for the new code. Yes, this risks not understanding edge cases. But not all of those edge cases are still important.
We typically talk more about "design" than "architecture", so for example our architecture is typically documented in "Design Docs".