The user URLs (or routes to be more "Drupal correct") are defined by the core user module (core/modules/user/user.routing.yml)
E.g.
user.register:
path: '/user/register'
...
user.login:
path: '/user/login'
...
user.logout:
path: '/user/logout'
...
user.page:
path: '/user'
...
So you can programmatically build links to these routes like so:
<?php
use Drupal\Core\Link;
// Replace user.login with other routes such as user.page
$user_login_link = Link::createFromRoute(
t('User log in link'), // The link text
'user.login', // The link URL generated from the route
);