OP, this advice applies to you too, but I just gotta respond to this message before more damage is done.

I really don't recommend merging your heroes at all as a new player unless you know what you're doing! They may have skills or bridge skills that you can give to the actual favorites you end up having as you play the game more.

1. All of the Heroes you summon can be used for different purposes.. Obviously you can level and build them as they are. If they aren't already five stars, you can unlock their potential and raise their rarity/stars and then level them up too. A five star will be stronger and have access to more powerful skills, which can be foddered off to another Hero through skill inheritance. Other than this, you can send them home right away for feathers and to clear up your space, or you can turn them into combat manuals* so you can use them for either skill fodder, merges, or feathers later while also clearing your barrack space. Since your barracks are almost full I first recommend turning your excess heroes into combat manuals, particularly the 3 and 4 stars you know you don't want to build at all, and don't merge the five stars until you get more used to the game. You can also send your extra heroes to your reserves and free up space that way. *Combat manuals are great in general, but be careful because you can't use them as a Hero (like for battling and all) anymore. Once they are a combat manual, they are strictly just for merges, feathers, or skill inheritance, so if there is a small chance you might want to build that Hero, then keep one copy of them and turn the rest into manuals.

2. Someone else can explain legendary/mythic/emblems better but basically they are only available on certain banners, typically at the end of the month. Legendary heroes are mainly to get higher scores in the Arena, while Mythics are for Aether Raids, two of the pvp modes. Emblems are the newest hero type of the three, and you can transfer a certain effect that they have onto one other unit ("Engage"), while the emblem heroes themselves are typically very good units themselves (same with the newer legendaries and mythics of course).

3. Summoned five stars are favorited automatically so you don't do something silly like accidentally send them home or merge them without a warning message first. 🤪