I’ve only done one international hike that I’ve had to really plan out because of how popular it was. I did the W trek in Patagonia, Chile and it gets booked up usually 3-6 months in advanced during high season.
They had limited designated areas along the trail that you had to stay in for the night in the park, so we had to book those in advanced. We were put on waiting lists and eventually were able to get the bookings for our trip.
The flights were another story, there wasn’t a direct flight down to the Southern tip of the country so we had to book flights separately. The first flight got delayed and we were worried it would affect us missing the next flight. Everything went smoothly in the end.
Anyway, for trips like that that are popular, I tend to do a lot of research into how long a trail is, how many miles I’ll need to hike a day, whether there’s running water, whether you need permits to camp, getting those permits, best season to go, what the weather will be like. I’ll start doing the research for a big trip about a year in advanced. Like right now I’m trying to plan for Kota Kinabalu for next year season.
I tend to carry the same gear for all hikes so I’ll usually have a pot too cook, gas canister, and that metal contraption to put the pot on the canister. I like the carry the least amount possible so I tend to bring underwear for each day but will probably wear same clothes couple days in a row (depending on how hot it gets).
I usually buy the food and extras like that nearby the hiking trail or in a nearby city before entering the park.
For regional hikes near where I live, I know what to expect and usually don’t do much planning. I’ll just look up where the start and end of trail is, the length and weather, usually the day before.