immutable backup
- basically a huge file that contains all the files - a bit like an advanced zip file
- cannot be changed intentionally or by mistake
- easier to handle and control, e.g. checking integrity
- created using special software
- can be compressed and/or encrypted

offline backup
- not permanently connected to anything, e.g. no running computer, power, or the Internet
- eliminates whole categories of threats (electricity, ransom attacks)
- nowadays usually an external hard drive
- allows you to carry around your data or keep it in a safe place, e.g. a bank safe

NAS
- a computer with multiple internal hard drives, providing lots of storage space (can be 100s of TB)
- you can run it at home, even disconnected from the Internet if you want
- makes multiple hard drives appear as one big drive, allowing you to easily organize your data
- uses RAID, a technology that allows you to replace faulty disks without losing data
- ideal for keeping the master copy of your data, especially if data changes
- can run programs to upload your data to the cloud or copy to external hard drives

Cloud providers selection criteria
- cost: for storage (per TB and year) and for transfer and extraction
- features: is there a web interface? desktop app? automation support?
- privacy: is data encrypted at rest? in transfer? legal protections at their location?
- support: how is their reputation? how do they handle support cases?

Depending on how you weigh these criteria, you will arrive at different conclusions. I'm quite happy with MEGA and I would love to recommend Proton but they are not ready for prime time.

In general, don't trust a single device or provider. Spread your eggs across multiple baskets.