You can do this on pc with veracrypt hidden partition.

On desktop/laptop I think it only is truly deniable with a HDD. Not sure about phone storage forensics but a Linux phone possibly could work for this. Might try for fun at some point

http://blog.nowherejezfoltodf4jiyl6r56jnzintap5vyjlia7fkirfsnfizflqd.onion/opsec/veracrypt/index.html

http://blog.nowherejezfoltodf4jiyl6r56jnzintap5vyjlia7fkirfsnfizflqd.onion/opsec/sensitivevm/index.html

Opsec level 3:Deniability

http://blog.nowherejezfoltodf4jiyl6r56jnzintap5vyjlia7fkirfsnfizflqd.onion/opsec/index.html

(Note: .onion links should be viewed with tor browser)

https://nowhere.moe/