I'm about to buy a device that has some SFP28 ports. I know that SFP28 is backward compatible with SFP+. But I want to know its backward compatibility with the SFP module. Can I use an SFP module (1Gbps) with an SFP28 port in all situations? (different implementation, different vendors,....)
There's no general rule.
Some SFP28 ports (25 Gbit/s) are backward compatible with SFP modules (1 Gbit/s), many aren't. You'll need to check the specifications for the device in question.
For reference, not all SFP28 ports are compatible with SFP+ modules (10 Gbit/s), and not all SFP+ ports are compatible with SFP modules either.