Personally I use a mixture of OS Maps, OS Locate, standalone Garmin eTrex GPS and OS/Harveys maps. Got a GPS watch a wee while back as well which I've been using in place of the GPS.
My uses are as follows;
Phone - Quick reference if I want an overview of the landscape relative to the map, especially in more challenging weather. I can instantly place myself in conjunction with double checking the physical map and the landscape features around me. OS Locate is a failsafe for quick GR's which I will check off against the GPS/Watch. Flight mode for the hills until I'm either set up in camp or back off.
GPS/Watch - Standalone units with dedicated batteries, provides me with all the route information I need including leaving breadcrumbs, marking waypoints of features or spots I'd like to revisit (photographer), potential campsites along the way, water sources not on the map on the day etc. I also love the altimeter feature (a bit iffy on the watch) as it also helps in placing myself / I can use it to give me a kick up the backside on a tough ascent / I can use it for off the cuff navigation. Easy to operate in heavy duty weather conditions as well when you'd rather keep your phone safe. I also use the watch or GPS to record my route, as I'd rather not eat into phone battery where possible.
Maps; Bread and butter navigation, the mainstay of all my navigational kit. Even when using my phone, I'll take bearings with the physical map and compass. They also give you a large field of reference and you can check / mark alternative routes, escape routes or the general lay of the land should plans collapse or change. I also make a point of getting the map out as it's something I love to do, it's immersive, allows you to understand the landscape around you (plus the tangible feeling of being able to read a map proficiently is very satisfying) and once home you can read it like a book. All of my maps are marked with main routes, alt routes, escape routes and points of interest - 1:25k and 1:50k. I'm a bit obsessed tbh, but it's part of the fun for me