Redirects are Brigadier's form of aliases and subcommands.

Below is how Minecraft makes /msg have an alias of /tell and /w.

public static void register(CommandDispatcher<ServerCommandSource> dispatcher) {
    LiteralCommandNode node = dispatcher
      .register(literal("msg")
        .then(argument("targets", EntityArgumentType.players())
        .then(argument("message", MessageArgumentType.message())
        .executes(context -> {... /* code ommited */ }))));
 
    dispatcher.register(literal("tell")
        .redirect(node)); // Alias 1, redirect to main command
    dispatcher.register(literal("w")
        .redirect(node)); // Alias 2, redirect to main command
}

The redirect tells brigadier to continue parsing the command at another command node.

Commands such as /execute as @e[type=player] in the_end run tp ~ ~ ~ are also possible because of redirects. Below is an example of a chainable command:

LiteralCommandNode<ServerCommandSource> rootNode = dispatcher.register(literal("fabric_test"));
LiteralCommandNode<ServerCommandSource> root1 = dispatcher.register(literal("fabric_test") 
    // You can register under the same literal more than once, it will just register new parts of the branch as shown below if you register a duplicate branch an error will popup in console warning of conflicting commands but one will still work.
    .then(literal("extra")
        .then(literal("long")
            .redirect(rootNode, this::lengthen)) // Return to root for chaining
        .then(literal("short")
            .redirect(rootNode, this::shorten))) // Return to root for chaining
        .then(literal("command")
            .executes(ctx -> {
                ctx.getSource().sendFeedback(Text.literal("Chainable Command"), false);
                return Command.SINGLE_SUCCESS;
})));

The redirect can also modify the CommandSource by use of a “redirect modifier”, which is used as a lambda.

.redirect(rootNode, context -> {
    // When redirecting, modify the looking direction of the command source.
    return ((ServerCommandSource) context.getSource()).withLookingAt(Vec3ArgumentType.getVec3(context, "pos"));
})