how does malloc understand alignment?

Alignment requirements are recursive: The alignment of any struct is simply the largest alignment of any of its members, and this is understood recursively.

For example, and assuming that each fundamental type's alignment equals its size (this is not always true in general), the struct X { int; char; double; } has the alignment of double, and it will be padded to be a multiple of the size of double (e.g. 4 (int), 1 (char), 3 (padding), 8 (double)). The struct Y { int; X; float; } has the alignment of X, which is the largest and equal to the alignment of double, and Y is laid out accordingly: 4 (int), 4 (padding), 16 (X), 4 (float), 4 (padding).

(All numbers are just examples and could differ on your machine.)

Therefore, by breaking it down to the fundamental types, we only need to know a handful of fundamental alignments, and among those there is a well-known largest. C++ even defines a type max_align_t whose alignment is that largest alignment.

All malloc() needs to do is to pick an address that's a multiple of that value.

I think the most relevant part of the Herb Sutter quote is the part I've marked in bold:

Alignment. Any memory Alignment. Any memory that's allocated dynamically via new or malloc is guaranteed to be properly aligned for objects of any type, but buffers that are not allocated dynamically have no such guarantee

It doesn't have to know what type you have in mind, because it's aligning for any type. On any given system, there's a maximum alignment size that's ever necessary or meaningful; for example, a system with four-byte words will likely have a maximum of four-byte alignment.

This is also made clear by the malloc(3) man-page, which says in part:

The malloc() and calloc() functions return a pointer to the allocated memory that is suitably aligned for any kind of variable.

The only information that malloc() can use is the size of the request passed to it. In general, it might do something like round up the passed size to the nearest greater (or equal) power of two, and align the memory based on that value. There would likely also be an upper bound on the alignment value, such as 8 bytes.

The above is a hypothetical discussion, and the actual implementation depends on the machine architecture and runtime library that you're using. Maybe your malloc() always returns blocks aligned on 8 bytes and it never has to do anything different.