In Fairy Tail, Erza can use Requip, but I wanna know how she gets her weapons and armor there in the first place? Her ability in Fairy Tail is not to put and leave items there, it's only for switching, so how did she place all her weapons and armor there in the first place?
Lets see what wiki has to say about this.
Requip is a type of Spatial Magic and Erza's signature form of Magic, over which she possesses great mastery. It is a type of Magic that allows her to swap weapons, armor and clothes at will. Her particular form of Requip is called The Knight.
Lets look at what spatial magic means.
This Magic revolves around the manipulation of space, which can allow the user to harass the asset for battle, travel, accessing alternate dimensions, and so on. This Magic is also used in conjunction for summoning concrete material or extraterrestrial beings to their stead, such as being able to conjure up armor and weapons or allow Celestial Spirits access to the human world.
This means her ability is to manipulate that space. So you're wrong to suggest she can't put items in that space. This means Erza can store, as well as take back armors, swords etc using her spatial magic. Her Armors disappear after being damaged supposedly back to the same space.
I would like to address that the size of the pocket Dimension is limited. So Erza can infact put weapons and the rest are stored in her house. Again from the wiki
Erza owns over 100 different types of armor, and over 200 different types of weapons.
This Magic allows the user to store weapons and armors in pocket dimensions and summon them during battle.[...]
The only known user, Erza, stated that there is a limit to how much the pocket dimension can store, and as a result, she leaves some of her armors in her room in Fairy Hills.
I don't know how Erza creates new or repairs damaged armors. From a different question it was answered
Erza is an user of requip magic, in other words, she can summon weapons and armors during the battles, but she does not create them.
Erza's Requip magic is where she stores armor and weapons in a pocket dimension and puts them on whenever she wants. Fairy Tail wiki direct quote:
This Magic allows the users to store items in a pocket dimension so they can easily summon it at any time, even during battle, which gives them a high level of flexibility in combat. However, there is a limit to how much the pocket dimension can store. Reequip can be used for simple or combat-related purposes. Some users may just choose to use it to change their clothing. More skilled users are able to Reequip different weapons in battle, and it is even possible to Reequip armors in the midst of battle, although the only Mages to ever do this are Erza Scarlet and Coordinator.
In my memory, I do remember that there was an OVA or a special episode or something where Lucy took a job where she dressed up as a cat girl and was looking for treasure for an old lady who turned out to be the person who raised Erza and some other children. In there, Lucy went to Erza's room in a scene and Erza showed her her other armor and explained her magic (my memory might be wrong though, but Fairy Tail wiki is correct). The special came out when there were only 77 episodes of Fairy Tail, I think.