Welcome to the world of music production! It. Is. A. Niiiightmaaaaare.....! But it is a lot of fun, which cancels out a lot of the negative
I know that track, it's been with me since they released it, it's a Max Martin production, and he's a great producer who knows how to get the sounds he wants.
Listening (again) to that song I'd say the chorus instruments (minus the obvious bass and drums) consist of
- the guitar from the start now strumming chords (EXS24 Guitars -> Acoustic Guitar)
- a piano doing simple on-beat chord thickeners (EXS24 Piano -> Grand Piano)
- another piano type sound doing a simple downwards 3-note arpeggio (EXS24 Piano - Grand Piano)
- an electric guitar strum like the piano from line 2 - (EXS24 Guitar - Electric Guitar -> Twangy Electic : put it through a guitar amp Amps&Pedals -> Amp Designer and flick through a few of the Clean presets until you find one you like)
- strings doing mid-range pad stuff (EXS24 Pop Strings - EXS Strings 2 is a good place to start)
- a higher more synth-ey choral sound doing pad stuff (Retro Synth - Presets-> 02 Synth Pads -> Electric Choir + some reverb is good starting point)
To get the Piano and Pad playing at the same time, select the Piano and Pad Instrument tracks and hit cmd+shift+d and select a Summing Stack. Now when you select the Summing Stack (name it something sensible, don't leave it at Sum 1), the MIDI signal generated from your MID keyboard are fed to both piano and pad instruments. You might need to drop the volume of the pad, but that's up to you.
I hope _some_ of that helps
If you want to write with these instrument I'd probably started with a piano+string pad sound so you get the attack of the piano but the sustain of the pad. Then you can get your basic chord structure down. From there you can add your little arpeggios and strums where it feels right, probably near chord changes and bar lines.
Late 2014 iMac 5K, 3.5 Ghz i5, 24GB memory - OS 10.14.6 - Logic 10.5.1 - Scarlett 8i6
Late 2012 MBP, 2.7 Ghz i7, 16GB memory - OS 10.14.6 - Logic 10.5.1