Yeah, Midjourney is by far the best in terms of the quality of the images and it gets better every month it seems.
Also yes, it costs money to use. You can try it out for free tho, look at part 1 to setup your account (You only really need a discord account). They say you have about 25 prompts to try out. You will most likely get more since it doesn't really look at the number or prompts but the amount of work it does for you. Just avoid using quality parameters and you will have some extra prompts to try.
Nevertheless, these principles should work in any AI-assisted artwork.
Yes and no. Every AI has its own way of seeing prompts and its own set of parameters and commands it can use, but if you only stick to describing what you want with text prompts, that will work on any AI.