These are different sounds. Ш is firm (like "sh" in english "sharp"), and Щ - a soft one. There is no similar sound in english, but it is close to combination of sounds "sh" and "ch".

Ш = Is like English (SH) Щ= You start it with SH and end it with CH but you should do it at once and fast. Ы put a clean pencil in your mouth and try to make the sound (I) like in (Israel) With a lot of practice you will be able to hear the difference between them.

Ш - SH Щ - softer version of "Ш") И - ee Ы - It's a strong sing, it tells us we should pronounce it a bit harder. ь - soft sing (it tells us to make the sounds a bit softer) I hope I helped you. If there's anything else I can help you, don't hesitate to ask me. Have a nice day.

Sorry for the late reply. Thank you so much everybody for your answers! All of your answers were helpful

Check out this video( just Ш and Щ). It's part of my Phonetic Course. https://youtu.be/1rJyDzDj3cY