With the CK out I tried to have a look at the equipment system for followers. There are some things that I want to change like equipment behavior:
- a caster should not wear any armor I give them
- a caster should change his equipment not only on armor rating, etc. (give someone an enchanted robe should replace the non-enchandted robe)

But I can't find the parts I need:

- Ingame we see followers with a default equipped bow. I can't see any reference in the CK. For example "HousecarlSolitude" (Jordis) does not have any bow in her inventory, not even in the default equip for her
- Am I right that there is no real section about equipment behavior. Yet there is a class selection but that somehow only determines stats (and therefore only things like wearing light/heavy armor or prefer 1H weapons over 2H)

Is it hardcoded?

Followers like housecarls has item sets, you need to modify that.
Simplest idea is to modify the houecarl item set to just contain clothing. or just remove the bow and shield.

An more advanced way it to give copy the iron armor into their inventory and have the item set blank.

Have a look the the housecarl sets. They dont include a bow but you maybe noticed they still use a default bow sometimes even if you remove all items from the NPC and let him reappear again.


That may help with the "default" equipment but
a ) not with the hidden wood bows
b ) that wont change their equip behavior. Any caster follower will still equip heavy armor ;(


Another problem I can't find a solution is the fixed levels the followers are (on first meet with the player). They scale as long as you not meet them. That means Whiterun housecarl Lydia become useless later.

You are right about the bow, it showed up on Lydia recently at level 47.
I was mostly thinking of the shield and armor.

leveling up is best done with an script, however I will definitely say my followers Lydia and Aela has leveled up, much more health than in the beginning.
The script could also just remove any hunting bows, but it might be better to find where they come from.

The bow & arrows appear to added via a script. I observed the following with Lydia:

Starting Condition: Thane of Whiterun, and I have not asked Lydia to follow me
1. Console: showinventory = No bow
2. Ask Lydia to follow me & showinventory = 1 Bow + 12 Arrows.
3. Ask Lydia to not follow me
4. Ask Lydia to follow me again = 2 Bows + 24 arrows.

I think that this is either a hard-coded behavior linked to a function in the scripts or there is a script somewhere that is providing weapons.

Edit:
Okay, so the bow & arrows are added via quest DialogueFollower, There is a quest reference alias labeled Follower which adds the bow and arrows. Since this is quest repeats each time you ask the follower to follow the bow/arrows increment.