gardening is a mess, i need help

[Senpai so Ecchi]

^title

can someone here pls explain how the heck gardening works?

i cut some growing grass in the wilds and get 1 seed from a random plant.
i go to the garden and it says i have collected 500 seeds.
why do i only have like 10 plants in different colors then?
what are all those seeds for??

and how the heck do i know which seeds i have not planted yet?
i can't tell those plants apart besides maybe "small round one, big round one, strange looking one, etc"

what i do is i erase my whole garden and plant all of them again 1 by 1 until something happens i guess.

this system is really poorly explained and i need some help pls.

(also, whoever thought using the "B" button to exit the entire gardening menu was a good idea, deserves a pay raise...)