How do I tell my 95 year old mom with dementia that we are taking her to a memory care facility to live? Or do we just take her?

I said nothing to my Mom until we got there. Then we told her she was going to her new apartment and would meet new people. I was one of the lucky ones, Mom acclimated to both her AL and LTC facility.

I've known many folks that have had to take their loved one to a memory care facility, and typically the family goes in ahead of time and gets their loved ones room all set up with some of their favorite things, and then they will tell their loved one that they're going to lunch.
The lunch of course is at the memory care, and after everyone is done eating the aides will offer to show your mom around the place by herself,(do not go along) and will eventually take her to her room while you and the rest of the family leave.
And it's often recommended that the family stay away for at least a week or two, to allow your mom to adjust to her new surroundings and folks helping her.
But of course you can call and check in with the facility by phone daily if you want to.
Please rest in the knowledge that you are now doing not what is best for your mom but also for your dad too.

She will be safer and your dad will be sad but in the long run it will be better for him also. To focus on himself and not his wife's needs all the time.

As for whats the best thing to tell mom. It probably depends where her dementia is. It sounds pretty advanced so I wouldnt tell her. Why get her upset in order to have to tell her over and over again and re upset her.

As far as happy strive more for your mom and dad being more at peace than happy. But even more important is her and your dad's safety

Best of luck to you and your family on this really hard journey

"Happy" is not a thing that's controllable within a person with advancing dementia. This is why so many require medication. Has anyone considered this for her? My 95-yr old Mom started the lowest dose of Lexapro 3 months ago and is doing better. There are lots of medication options and everyone is different, so it may not happen on the first try. Her wandering makes her an appropriate candidate for MC. Please consider using a therapeutic fib when transitioning her. Telling people with dementia "the truth" is counterproductive because they no longer have reason and logic and therefore cannot process the change. This is also why meds before she goes would be merciful.