How do you deal with the silence and not talking much?

How dreadful it is that your grandmother's children left YOU with this burden to care for her, with such a terrible dementia diagnosis, all on your own, w/o any help and with no escape.

Like AlvaDeer said, I'd give your Aunt your 2 week notice IMMEDIATELY and get out of this situation at once. Not for lack of love for your grandmother, but for sheer burnout and the absurdity of being left in such a situation in the first place. 6 years is 5 years and 11 months too long that you've had this job with no pay and no benefits, so it's time you strike out on your own and find a job that you're paid to do. It's your turn now to develop a social life, find friends and start dating!

Wishing you the best of luck moving out and on with your own life now. You deserve to.

Carry on conversations with grandma just as you normally would.
My Husband was pretty much non-verbal the last 5 years or so of his life. he made noises but never spoke. (He did say "ouch" once when he hurt his hand)
Can you get involved with some things that you can do from home?
Tutoring
There are online support groups.
The senior center near me has a program where they have people call Seniors to check in on them, make sure they are ok and just have a chat to keep them company.
Any of these might keep you busy, have an outlet for yourself.
It is difficult. I do hope you have more help than just weekends.

Karn, my heart goes out to you, but you should not be doing this. You Grandma's children need to take on responsiblity to have her placed in care and you need to get on with education (if not done) with job, with your own place and your own life. I would encourage you to consider "giving notice" and doing this. I recognize that doing this may give you a place to stay and enough money to stay alive and have a place to live with food provided, and I recognize how crazy scary it will get to leave what you know and get out there in the world we enter when you are grown, but it is time to consider doing just that so you don't sacrifice your entire 20s, that time when you get out there on your own, get a friends and a life, and begin to mature. I surely do wish you the very best.
I guess some of would say it is "good luck" to have a grandma who is content with quiet as opposed to the difficulties that hit many elder with Lewy's (tho it leaves YOU with no real communication at all).
My brother had a probably early Lewy's Dementia diagnosis, and he died before it could get bad of another cause. To be frank, I think he was lucky; it was what he wanted; he knew what he had and feared where it would take him.
Good luck and best wishes out to you.