Check this guide to find all the branching oaths and extra level in Star Fox Zero!
Star Fox Zero lets you take multiple paths in certain levels, like in previous Star Fox games. Many of them are not unlocked until you go through the game once and require going through levels as a different vehicle.
I'm going to help by explaining which levels provide these branching paths, and what you need to see each one.
These are levels that you must play to unlock a vehicle type, which is required to do certain branching paths.
When you start the game, you will go through each level like normal, at first, until you reach Sector Beta, the 5th level. There are spoilers ahead.
When you back through Corneria with the Walker unlocked, you can transform to the walker on land during the part where an ally says, "We're taking the coastline".
This takes you to the alternate boss level Aquarosa. When you beat this boss, you unlock an alternate paths on Area 3 and Zoness. You have to beat Aquarosa twice if you want to play both paths.
This time you are going through with the arwing and walker transformation and you have a 7 minute time limit.
You play this level going through the base this time and you eventually see Andrew, though you can't attack him. Beating this version of the level unlocks the Asteroid Field.
You must shoot your way through the asteroid field while protecting Great Fox from enemy ships and drones that latch onto the ship.
When you have access to the Gravmaster transformation, you can get the branching path on Titania.
This unique boss fight has you playing as Peppy as you take down 14 big guns on the Salvadora ship.
Some of the levels will have branching Star Wolf paths that have you fighting someone on their team. As you unlock more branching paths, these Star Wolf events appear on certain levels.
The Star Wolf event has Pigma coming in, but you don't fight him yet. Just lock-on with the ZL to progress to the next part, which unlocks the Wolfen boss level.
Just take out Pigma and Andrew. When you damage them enough they each use a special attack, but they are easy to beat. When you beat this, you unlock the Great Fox level.
This fight is nearly identical to the Asteroid Field level. At the end you fight the bird boss from Fortuna.
Star Wolf shows during the level and you have to lock-on just like Pigma in Sector Alpha. This unlocks the Hunter Boss level.
This is a solo fight against Wolf.
That's all of the branching paths and extra levels available in Star Fox Zero. Let me know if you have any questions!