Tadashi hangs around as a spiritual guide, but only Baymax can see him because once Baymax’s scanners have someone in their database and they havent moved on, he can see and scan them. The scans are different for spirits, just aura readings.
This is why he kept saying Tadashi is here, also along with the meaning that he is in their hearts. He soon realized that he was the only one to see him, so he tried to take a photo when Tadashi was with the group. Showing Hiro and after explaining, Hiro of course just shuts down. His brother was there all along?
He can still talk to him even though he cant get a verbal response, only pictures? Oh God… did he see him almost kill Callaghan? What did he think of him now? Lotsss of emotion and regret, slight panicking until Baymax shows a photo that was just taken seconds ago showing Tadashi holding Hiro tight, so glad he can still be apart of their lives.
They continue to use pictures to hang out with the gang, hang with Aunt Cass, say hi to Mochi even though she follows him around the house confused, help Hiro with work, see his thoughts on their Hero work (he isnt happy at first but slowly comes around), etc. It isnt the same but they are super grateful for Baymax so they can have this.