Why can't I use Indian rupee symbol (₹) with Arial font?
This is because macOS Mojave uses Arial version 5.01.2x, which includes the following list of characters:

! " # $ % &amp; &amp;#x27; ( ) * + , - . / 0 1 2 3 4 5 6 7 8 9 : ; &lt; = &gt; ? @ A B C D E F G
H I J K L M N O P Q R S T U V W X Y Z [ ] ^ _ ` a b c d e f g h i j k l
m n o p q r s t u v w x y z { | } ~   ¡ ¢ £ ¤ ¥ ¦ § ¨ © ª « ¬ ­ ® ¯ ° ± ² ³ ´
µ ¶ · ¸ ¹ º » ¼ ½ ¾ ¿ À Á Â Ã Ä Å Æ Ç È É Ê Ë Ì Í Î Ï Ð Ñ Ò Ó Ô Õ Ö × Ø Ù Ú Û
Ü Ý Þ ß à á â ã ä å æ ç è é ê ë ì í î ï ð ñ ò ó ô õ ö ÷ ø ù ú û ü ý þ ÿ


Indian rupee sign is not on this list, so TextEdit uses PingFang SC, which is a system fallback font introduced in OS X 10.11 El Capitan:


  OS X 10.11, aka El Capitan, comes with a new system font for Chinese users, named PingFang, it includes 6 weights for both Simplified and Traditional Chinese. The same font also appear on iOS 9 as the default UI font, though Apple didn't mention it explicitly.


No single font contains every possible character.  To see which ones a particular font does have, go to Applications > Font Book, select the font, and do View > Repertoire.  To see which fonts do have the rupee symbol, open the Character Viewer (Edit > Emoji &amp; Symbols), select the character, and look at the Font Variation pane.