In need of up-to-date Navigo Easy card info

It seems every time we go to Paris, the powers that be are messing with the public transport ticket system. (I will miss those cute little t+ paper tickets). Well, I thought I had the new system figured out for our April/May trip to the city, but alas, the more I read, the more puzzled I am. My wife and I will use the metro, the RER and even a Transilien train once. We will use all those modes sparingly because we love walking through the city when time and weather permit, so my original plan was buy a Navigo Easy card, load it with X number of metro tix and x number of bus tix and even a Roissybus ticket to get to the airport at trip's end. But now, I am confused about whether those different modes of transport can be loaded on the same card.
It doesn't make sense to me that metro and Bus tickets can't be loaded on the same card and the system can't sort which transport I'm on as I tap in. If they aren't on the same card, and I transfer from Bus to metro, that would mean I would have to use a "Bus" card and a "metro" card. Doesn't seem too efficient to me. Hope my confusion is unwarranted. Oh, and I have perused the RATP and Ile de France mobilitie websites and clarity is not their strong point. HELP, PLEASE, fellow Forum experts!!