In a lot of RPGs, you can learn new abilities by earning XP and leveling up your characters. However, things work slightly different here. You should instead update your Base of Operations, which leads to new abilities. For instance, you can construct a new wing for your Medical Ward, and that will grant access to a new healing skill. You might also gain a new perk in the process. You still want XP, however, because gaining levels allows you to use better weapons and gear.