This room can be a bit hard to parse at first (like many in this hold). The pressure plates in the middle are used to allow briar growth around the edge of the room. You can only hold down three plates at a time, so this may seem impossible, but there is a mechanic that helps here. Briar can't grow the wrong way across a force arrow, but once a root is connected to briar that crosses over a force arrow, it ignores the force arrow and can contribute to that briar's growth.
So the briar root in north nook is the only one that can contribute to growth originally since it is the only one that isn't blocked by a force arrow. The three yellow doors along thet briar corridor that are closest to it must be opened for it to grow as much as possible. That means Beethro, the mirror, and the aumtlich must hold down three of the pressure plates in the middle of the room for a long enough time for it to grow. Once you get an arrangement that holds those plates down for long enough, the briar will grow to connect up with the root on the est end of the room. At that point the eastern briar root is connected to the briar and you can ignore the northern one from now on as well as all the yellow doors between them.
Your goal now is to open the yellow doors between the eastern root and the southern one until the briar connects to the southern root. When you finally have that accomplished, you can again ingore all previous yellow doors and just open the doors between the southern root and the aumtlich.
I say "
just"
but the manipulations involved in holding down the correct pressure plates can get pretty tricky here. Please let me know if you need specific hints for any of the steps above.
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