Let’s take the Unix V6 source code for example. The linux one is not found. Take a look at the userstructure

struct user
{
  // ...
  char    u_uid;  // 实效用户id(effective user id)
  char    u_gid;  // 实效用户组id(effective group id)
  char    u_ruid; // 实际用户id(real user id)
  char    u_rgid; // 实际用户组id(real group id)
  // ...
} u;

Take effective users and actual users as an example. The actual user has been determined when logging in to the system, such as ubuntu用户的id是108，www-data的id是105。若用ubuntu用户运行程序，则实效用户为108；若用sudo命令切换到www-data身份运行，则实效用户为 105，而实际用户仍为108，所以$USER还是实际用户ubuntu.