There are lots of better ways to make a shower pan these days. The old and typical big builder rubber membrane and cement method is for many people out of date and tends to fail. Grout between ceramic tiles is after all, not waterproof and it's only a matter of time that the rubber membrane ends up full of wet cement. I get it though,,,,, why it is done,,, because it cost less than better methods.
But, if you plan to stay in your home for a long time or if you have the budget and just like doing things the best way possible then there are foam shower pans, Schluter systems, etc. that you may want to consider if you want a shower system that will never fail.
https://www.youtube.com/watch?v=LeDld1ls_6c