What is exactly feature apps using icloud?

Hello there!

I am not sure for taking right place to ask, maybe that's needed to ask directly from developers of each app, so sorry if this is off topic.

But iam very confused about 3rd party apps in this feature.

I have no this question about native apps, there us all clear and also mentioned in support articles, also most obvious.

But what about other app? What would happen and sync if turned on?

Because for example: iam totally sure that TG data would be not synchronized

(the reasons are:

1. first of all that's security issue and can't be by design;

2. it's proved by for example with android app so it is not backed up and sync data in anyway 3rd p remote services;

3. it's has no sense at all for it, because all settings and data (except data from security chats that end to end encrypted by hardware level with installed app and can't be synchronized by design to another hardware);

4; also if it will be anyhow sync or backup data to 3rd p remote that's will be absurd amount of storage and traffic -- *and that statement i can also extrapolate to other things like Mega; One drive; and others. *

the TG was just first and common example, so is any almost other apps have sync the settings that can be synchronized already and no need 3p interaction)

So please can you explain me and clarify about that feature. If you know.

Thank you!