So, you've defeated Sahelanthropus and have watched the credits, but you noticed something after the credits were finished. Yes, Mission 31 was just the first chapter of the game and there's another chapter waiting for you. However, things play out a little differently with the missions in Chapter 2, as there are plenty of them that are repeats of ones you've already done, but with a specific ruleset in play.
There are three different rulesets for these repeat missions, with each one being in the title of the mission. They are Subsistence , Extreme and Total Stealth . All of these variants make the Chicken Hat and Reflex Mode unavailable. Extreme makes it so enemies deal more damage to you. Total Stealth makes it so that if enemies are alerted (the red alert), then you automatically fail and are sent back to the previous checkpoint.
Subsistence makes it so that you start with nothing and can bring no weapons or items along. Also, you cannot bring your Buddy nor call for support, either via supply drops or support fire. You are completely "naked" and everything you use in the mission has to be found or collected from enemy soldiers. This means that you'll need guns dropped from enemies, which likely means you won't have access to suppressors and other easy non-lethal means.