For most SMS apps, not messaging services in app form like WhatsApp Signal or Telegram, messages are unrecoverable by users. I say most because some regular SMS apps like Next SMS offer built in backup services, but for Google Messages the only option for recovery is if there was a backup created. Same for attachments.
Here is a similar question as yours posted at
Google Support.
As for attachments specifically, the pics show up in your Messages (media) folder that is viewable by both the Gallery app and Google Photos app but you're correct when deleted they do not appear in the trash of either of these apps because they were not controlled by the app only viewable through them. I equate it to deleting something off of an SD card on your PC. You can use Windows' File Explorer to navigate to and view items but when deleted they don't show in the PC's trash because they were never "on" the PC. These images were never "in" the Gallery or Google Photos, so... As for where do the files go, I would imagine they just sit there unrecoverable until they are overwritten.
I found this similar question at
Google Support.
Are they recoverable by other means? I don't know for certain, I've been lucky enough to not have had to go down that road. I will say there are a number of apps and services advertised online that claim to be able to recover deleted data, media. Do any of them actually work, I'm not paying to find out, but I doubt they would work in this case only because I'd like to think that Google is doing some encrypting. A recovery program might find the data and be able to point to all of the bits of that data but won't be able to construct anything recognizable from it due to the encryption. And if not Google, then your device also encrypts data, with the same result.
This what Gemini says about Google Messages using encryption.
Yes, Google Messages can be encrypted using end-to-end encryption, but only when using RCS (Rich Communication Services) and when both users in the conversation are using Google Messages with RCS chats enabled. When end-to-end encryption is active, messages and their attachments are encrypted, meaning only the sender and receiver can read them.
Your post sounded like you were just asking the question and not like you were actually trying to recover something. Was there more?