There's an app I can use that will do all the hard work?
Seriously, I use maybe a dozen apps when I'm away on my bike and not all are cycling specific.
I highly recommend
https://cycle.travel/map for route planning. There is an IOS app out and Android is on the way. It is the best bike route planner I have come across in terms of picking quiet, lightly trafficked routes and has a host of useful features specifically for multi-day bike adventures.
You'll want to have a look at the dedicated Camino apps for accommodation etc.
A useful tool is Osmand, not so much for it's bike routing capabilities - it can be erratic - but for the layers and layers of information it contains from gradients to shops to water fountains. It has the advantage of being personalisable and works off-line. A great back up. Although not the easiest to master.
You may want to consider a weather app. The good ones can be tricky to master and be useful.
Google Maps (absolutely NOT for planning routes) has the ability to "save" local maps so that the information on them is available offline. Mark off albergues, shops, interesting things to see, download the map and hey presto! they are on your phone even without a signal.
If using a gps or other journet recording app (Strava, RWGPS) you may be interested in keeping a daily log of your ride. Photos can be added from your phone with the press of a button an with a bit of internet there's a record of your route and all your photos. Both Strava & RWGPS offer this, Strava, at least, on a free sub.
Whatever you choose, it makes good sense to be familiar with how your chosen apps work before you leave. They all have foibles and it's better to know them in advance.
And, as with any routing tool, just because the computer says to go this way doesn't mean that we should! Weather, road works and lots of other things can come in to play. Always a good idea to have a reliable back up. And use some common sense.
Oh! A route is just a line on a map. There's no law that says it has to be followed slavishly!
Buen Camino