A famous example of a deaf/blind person is https://en.wikipedia.org/wiki/Helen_Keller - this article contains some explanations about how she was able to learn to communicate with the world.
Look into Hellen Keller and how she learned. I'm not 100 percent brushed up on it but I do know it involved touch and feeling. The teacher would trace letters onto her hands and also use temperatures and textures to convey meaning to her.
The teacher would also out helen keller’s hand on the teacher’s face so helen could feel throat vibrations, the way the lips moved. Because of that, helen could actually talk (even though it sounded a little strange). You can youtube her speech
I've spoken to someone who is deafblind before. They had an interpreter that would fingerspell onto their hand by touching it in certain places and making gestures. They would then fingerspell their answer back to the interpreter, and the interpreter would speak what they replied.
A lot of people don't know that you can use computers and phones with a refreshable braille display. Some people who are deafblind can also read braille, so you could send them some text to their phone/computer, and then they should be able to read that text with a refreshable braille display. They then could reply to you by typing on their normal or braille keyboard, meaning you'll get a reply in text.
For fast, real-time communication, an interpreter is required. It's conceptually not that different than communicating with someone who's deaf - you speak to them and their interpreter signs, then they sign and the interpreter speaks. The only difference is that they're feeling signs rather than watching them.
But when the communication isn't real-time, in-person, then many deaf-blind people can text and email using a refreshable braille display - so if you're not in the same room you'd never know the text / email you're getting is from someone who's deafblind.
There’s a few different ways. If the person is mostly/fully deaf and blind, then communication is primarily through touch sign language interpreter (signing on the person’s hand) or a device that you type into that outputs braille for the deafblind person. Some deafblind people are not fully deaf or blind and can see sign language if the interpreter is very close to them.
The DeafBlind community has used a variety of tactile systems to communicate in the past, but relatively recently, as more opportunities to communicate with each other have arisen, Protactile ASL is the language of choice for most people who identify as DeafBlind.
I have actually dealt with a partial deaf blind student back in elementary school.
You can sign ASL into their hand so they can feel what you're signing(normally individual letters). Or they hold your hands to feel the gesture/sign you're are making.