Software build rules are placed in a so-called "skel" (short of "skeleton"). By default, each skel is just a shell script, telling how to get the source code, how to configure, compile and install it. If there is any architecture-dependent issues, then you just write necessary if-s in that skel.
The most crucial decision is installation destination of the skel. In
too many cases it can not be some temporary directory you create during
the build. Because much software tend to hardcode various configured
installation paths and it just won’t work from other place. That is why,
like Nix does, skel is installed to some permanent path. If you
have got enough RAM, it could be just a directory in tmpfs.
Actually skels are not shell-scripts, but
redo targets. So they could be written
on any other language, even be compiled programs. redo is used
because of its built-in locking capabilities and ability to run jobs in
parallel.