Japt, 10 bytes

;BíC ËpUèD

Try it

;BíC ËpUèD :Implicit input of string U
;B :Uppercase alphabet
 í :Interleave
; C : Lowercase alphabet
 Ë :Map each D
 p : Repeat
 UèD : Count of D in U

Or, using the Vyxal approach, which feels like cheating to me ...

7 bytes

á æ¶Uñv

Try it

Japt, 10 bytes

;BíC ËpUèD

Try it

;BíC ËpUèD :Implicit input of string U
;B :Uppercase alphabet
 í :Interleave
; C : Lowercase alphabet
 Ë :Map each D
 p : Repeat
 UèD : Count of D in U

Japt, 10 bytes

;BíC ËpUèD

Try it

;BíC ËpUèD :Implicit input of string U
;B :Uppercase alphabet
 í :Interleave
; C : Lowercase alphabet
 Ë :Map each D
 p : Repeat
 UèD : Count of D in U

Japt, 10 bytes

;BíC ËpUèD

Try it

;BíC ËpUèD :Implicit input of string U
;B :Uppercase alphabet
 í :Interleave
; C : Lowercase alphabet
 Ë :Map each D
 p : Repeat
 UèD : Count of D in U

Or, using the Vyxal approach, which feels like cheating to me ...

7 bytes

á æ¶Uñv

Try it