Multisite setup

Hi there. I’ve read a couple of threads here about multisite but without answers.

Does it activate at network level or only per site, and if activated at network, then can it be overridden per site?

I want to use Amazon SES in a multisite with a default configuration that applies to all sites. But I would want to be able to override this for a given site if necessary. Currently I use Branda, but that doesn’t let me use the SES API, only SMTP.

Copilot tells me I need to activate Fluent per site, but that I can use SMTP credentials that I would place in the wpconfig as a preset. And that these can then be optionally be overridden by per-site SMTP settings.

That would be ok. But can I instead preset API credentials? What if I activate Fluent at Network level or would that preclude me overriding it per site later on?

Any advice is appreciated, thanks!
Rob