This is probably the most commonly asked question on this forum.
The topics for both the practice and real thing will both be from the official guideline. As per how similar the practice replicates the real exam difficulty; that can(and has) been debated endlessly. Google reddit MCAT compilation of commentary for the new exam. You'll get tons of reviews and thoughts answering this very question. Additionally there is a spread sheet that has a list of hundreds of peoples practice tests scores compared to the real thing. Small sample size and all but still n=300 is alot better than n=0 or n=1.
One sentence cliffnotes answer to your question; a large proportion of people who took the AAMC practice test scored within 2-3 points of their real exam score. Some did better on the real thing. Others worse. Getting 80+% on all the sections is a pretty good sign you are ready to take the test.