What is seedlings in the context of subeta? What can you do with them? Is there a guide for this? I looked it up on the subeta fanwiki and there is only a basic description with nothing more elaborated

Seed items from the Seedling shop can be planted in the Garden to turn into plants.

There is a List of usable seeds at SubetaLodge.

After you plant the seed, reload the page and you will get an option to water the seed. You will be able to water again after 24 hours (not after midnight), so if you plant a seed at 1pm Subeta time, you won't be able to water again until 1pm the next day. If you miss watering for a whole day, your plant will die. You need to care for your seed for 3 days and then it will turn into a plant. You can then feed the whole plant to your pet, or "use" the plant to get some other items to feed your pets.

NOTE: if you find an and plant it, on the third day when you pick it, the plant will "attack" your active pet and the pet will turn into an aeanoid of whatever color your pet is:

You mean the shop? It sells seeds and plant-related items. You can plant seeds in the gardens in Veta. Since you're new, check out the FAQ and SubetaLodge (usually far better than the wiki). c: