Telegram – Difference Between a Channel and a Group

Yes, it's 1.5GB per person per file, regardless of the chat type.

Maybe the sentence you quoted wasn't clear enough:

you can send files up to 1.5 GB

• Generally, you can send in Telegram

AND

access them from any of your devices

• i.e. not only from the device you sent them

Another way of writing this, that maybe would've been more clear to you about that, is:

Since Telegram's launch in 2013, you can:

• Send files up to 1.5 GB
• Access your files from any of your devices, including computers.

Why/what is it good for? Because it's perfect for everything from studying to sharing personal archives.

You can only change the type of a channel (public <=> private).

If your channel doesn't have thousands of users, just create a supergroup and share its joinchat in your channel. (and also you can manually add those who haven't changed their group privacy...but it takes time because of the rate limit of telegram APIs)