没有default.target，systemd如何启动？

systemd 文档和各种在线资源都声明 systemd 应该（默认情况下）启动名为“default.target”的“特殊”目标。可以通过 systemctl set-default 更改此设置，这基本上是从 default.target 到所需目标的符号链接。

但是在我的系统中没有这样的文件。没有/etc/systemd/system/default.target，没有/usr/lib/systemd/system/default.target（实际上这个文件在整个系统中是不存在的）。系统仍在启动。我的问题是怎么可能？

（当然我可以自己做一个符号链接，我只是想知道如何在没有 default.target 的情况下定义一个默认值）

附加信息：

# systemctl get-default
graphical.target

所以我的默认目标是graphics.target。但它在哪里（以及如何）定义？

# find /usr/lib -iname "default.target" # no results
# find /etc -iname "default.target" # no results
# lsb_release -a
LSB Version:    n/a
Distributor ID: Gentoo
Description:    Gentoo Base System release 2.4.1
Release:    2.4.1
Codename:   n/a

# systemctl list-units --type=target
UNIT                  LOAD   ACTIVE SUB    DESCRIPTION                  
basic.target          loaded active active Basic System                 
getty.target          loaded active active Login Prompts                
graphical.target      loaded active active Graphical Interface          
local-fs-pre.target   loaded active active Local File Systems (Pre)     
local-fs.target       loaded active active Local File Systems           
machines.target       loaded active active Containers                   
multi-user.target     loaded active active Multi-User System            
network-online.target loaded active active Network is Online            
network.target        loaded active active Network                      
nss-lookup.target     loaded active active Host and Network Name Lookups
paths.target          loaded active active Paths                        
remote-fs.target      loaded active active Remote File Systems          
slices.target         loaded active active Slices                       
sockets.target        loaded active active Sockets                      
sound.target          loaded active active Sound Card                   
swap.target           loaded active active Swap                         
sysinit.target        loaded active active System Initialization        
timers.target         loaded active active Timers                       

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

18 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.