I was a flip phone holdout until late 2017, I have since become utterly dependent upon my smartphone (an iPhone SE, the last of the pocket-sized phones), and all of my international trips would have been somewhere between difficult and impossible without it and a few apps. I am also utterly dependent upon a few pieces of desktop software and web pages.
Maps.me is a convenient way to use OpenStreetMap data offline for both mapping and route-finding. The maps are usually good and reasonably complete, even in small South American towns. The route-finding is quirky, though. Sometimes it will not see roads that clearly exist on the map; other times it will suggest utter nonsense. While it claims to be able to plan routes for cars, bikes, and pedestrians, the car routes tend to be the least nonsensical. It can usually be trusted for short distances within cities, but you should always double-check longer routes.
This has been my go-to offline mapping program for several years. It offers offline USGS/USFS topo maps within the United States, and usable computer-generated “global topos” elsewhere. These topos can be downloaded either for the area around a peak, or for that around an arbitrary GPX track. The app also keeps track of peak-bagging statistics, for people who play that game. However, its offline maps have always been unreliable to download over slow connections, and have recently stopped working for me entirely. I am currently looking into other options, and have high hopes for the CalTopo app currently being developed.
I do all my route-planning with CalTopo, which has a variety of maps and satellite images for the whole world. I used the free version for a long time, but recently became a subscriber, partly to support the project, and partly to eliminate some restrictions on the app. While the iOS app is still a work in progress, I hope to someday use it to replace Peakbagger for my offline topo needs.
Windy has horrible, useless graphics on both its website and mobile app, but I use it anyways because it provides worldwide point forecasts from three different models: the European ECMWF, considered the best in the world; the American GFS, which is also good; and Meteoblue, which focuses on mountains and has higher-resolution models for some ranges (e.g. the Alps). Meteoblue’s summit temperatures seem to be most accurate, while ECMWF may be better at wind speeds. Comparing the three models is a good way to get a rough idea of forecast reliability: the closer they agree, the more you can trust the forecast.