It seems that most outside activity is travelling to and from an interior location.
Are most encounters do inside a location?
Is that just my misconception or is it the usual case?
Depends on how comfortable I am with a system. A dungeon is a great place for a new GM. It's also a nice play for an experienced GM learning (or play testing) a new system. Reduces the variables considerably, and allows me to focus on the basics. The same thing applies to players. So when doing something new, not only is it nearly all inside, it is primarily a dungeon-like complex (as opposed to, say, several buildings in a town). I'll branch out to a more varied approach from there.
Then it becomes more driven by the nature of the adventure. I've had multi-sessions of wilderness travel, or all town, or all dungeon, if that is what the characters are pursuing. I try to provide a variety of options so that we aren't doing the same thing all the time, but you never know what players will do.
I do not do the "1 token encounter between site A and site B" thing. The wilderness is the wilderness. Careful characters could make it though a relatively safe area with no appreciable encounters--not even the "saw it over the next ridge and avoided it" type. Careless or unlucky characters could have multiple wilderness brawls and then run for it or have a TPK. I had one adventure a few years back that was all about the survivors trying to get out after getting pinned in a valley by overwhelming forces guarding the passes, weeks from civilization. They tried to take a short cut that multiple NPCs had warned them was dangerous. They were getting off light due to luck until they weren't, which had allowed them to get in too deep. The world is what it is.
It varies drastically depending on what the adventure is about and what sort of trouble the PCs get themselves into through their own curiosity and/or stupidity. Some adventures I've run take place entirely outdoors, even when there's maps with various places of interest or strategic locations that PCs could use, such as when assaulting a camp of hostile creatures, like gnolls, orcs or ogres. I might have a map with various huts or areas between the mountains and what not, similar to how indoors maps have different rooms and such. But the battles are mostly outside, unless the PCs need to enter a hut or something to rescue some prisoners or exterminate some leftover enemies hiding inside. Other times the PCs go inside a cave and it's almost all indoors. It just depends on what they're doing.
Varies depending on where the PCs are, or where they are going.
The X part of BX introduced wilderness encounters and there was the module Isle of Dread which is pretty much wilderness encounters everywhere just waiting to happen.
If a campaign is mostly overland oriented and dungeons/ruins are few and far between then its more likely there will be more stuff going on outside than inside.
Dragon Storm pretty much runs on the outdoor encounters as there are just short of zero indoor ones. Pretty much everything happens either en route to a locale, while camped, or at destination. Sometimes all of the above.
Generally, inside. Whatever is living there is occupying an enclosed area, generally much smaller than open wilderness. Unless we're talking about something like a tomb that's been sealed for a thousand years, in which case there might be no encounters at all. The difference in how densely a building is populated versus open land is reflected in the time scale and how period wandering monster checks are.
Generally about 2/3 outside and 1/3 inside. The wilderness is wild and interesting; it's a shame to just ignore at you walk to the next dungeon. Also a good size fraction of my adventures don't have a dungeon at all.