Trying to decide whether to use an on-line Password Manager or Apple's Password App in settings, in Sonoma?
I went with Apple's one. Does all I need. Your needs might be different. What is undeniable is the convenience of having all my passwords on all my devices.
If I use Apple's, does it automatically have to be in the cloud, which I would rather it not be.
If you want to sync across devices it will have to be in the cloud somewhere. So, if you use an "online Password Manager" as mentioned in your query, that will also be in the cloud. Apple's syncs via iCloud. I'm confident that this will be more secure than one I might cook up myself.
Or, maybe just creating a password protected .dmg file on my computer, with a password protected numbers file is the better way to go.
Maybe, or maybe not. Not much good if you want to sync across devices, nor if you want your Mac to input passwords for you. Again, it's down to your requirements, and only you know those.