Yeah I don't know any apps that have custom notifications out of the box these days, apart from Messenger. It's too convenient just to use the operating system's API and default to the user's default sound.
I don't know if there's some way to override/customise the notification sounds on Android on per-app basis. At one point in history I wanted Twitter to go [random bird noises] and Reddit to go "Le." but since I don't use either of them anymore I really didn't investigate this further.
In WhatsApp you can set custom notification sounds, I believe it even supports notification sounds on a per-contact basis. So safe to say you can do it as an app developer.
I used to use custom sounds I downloaded for a few people and it was useful, but it's clunky and time-consuming to set up for even a few contacts. I had to get the sounds on a PC because all the sound apps on the store feel like malware and are riddled with ads.
It's actually really easy for a developer and for a user. Apps can provide a notification sound for themselves, but the Android API automatically creates a user interface inside of the notifications settings that let a user set a sound of their choice on a per-notification-channel (for example, Discord has separate channels for "Incoming calls", "Direct messages", "Friend requests", and a heap more) basis.
On my Samsung Android phone it's configurable but I think when I installed the app aeons ago it used their distinctive ding. On desktop Linux incoming messages throw standard push notifications.
I'm wondering whether it automatically copied using the default notification from my old phone then. I'm pretty sure I didn't change the Signal Notification sounds on my current phone.
I'm boomer enough that I don't trust buying things (apart from fast food...and tap to pay in physical stores) on my phone. Always go to my computer before shelling out money. So I have never experienced Ebay on my phone.
(I'm also a young millennial, not an actual boomer.)