What is the difference between IDF app and Bonjour RATP app? It seems you can buy metro tickets on both.
That is a good question. I have perplexed about the IDF Mobilité app ever since it was issued as the Bonjour RATP app had been in service already for some time. Why two official apps?
Generally, I find the RATP website confusing, poorly organized, and very difficult to extract usable information. It's structure is simply bizarre and meandering. Conversely, the IDF Mobilité website is much better organized and information is presented in a much more understandable and logical format. Thus, I use the IDF Mobilité app, and it does it all. It is as good as Citymapper, plus it can be a Naivgo card replacement. I have less experience with Bonjour RATP, in fact I removed it from my phone.
If you are just visiting Paris for a few days, Citymapper works very well and it works in most major cities. If you like to keep apps to a minimum, just use Citymapper.
Googlemaps? I use googlemaps for walking around and quick restaurant/café recommendations. Googlemap's information about public transportation is inadequate and incomplete. It is not a replacement for any of the above mentioned apps for those using public transportation.
I use RATP out of habit, but there is no practical difference between the two when it comes to ticketing.