A "memory model" and "memory management" are two different things.

Memory management normally conotes the process for allocation and reclamation of memory in a program.

A memory model defines where reads can get their information from in a program. In most contexts that is informally "the most recent write" in terms of wall clock time (i.e. sequential consistency).

In C++, C and Java, this is not necessarily how things are. For example in Java writes can be buffered preventing reads in other threads from seeing them right away. This means those reads will get a value from an "older" write (again wall clock time).

It's all really bad for reasoning about your program :(