This one's pretty cool too. UI/UX Design is awesome once you start understanding why are apps designed the way they are and why is the user experience given so much importance.
Now, there are quite a lot of paid courses which are good in general but seem to be a little too expensive.
Don't worry though, YouTube for the save again!
There are quite a few software which are really good.
Software:
Out of the three, I personally prefer Figma because it's really just opening the browser and going to the website.
It depends on the OS you have too, I believe Sketch is only available for MacOS. You can google the names of the software to access them.