What happens when a FSMO holder goes down?

Let's say I have 2 DCs and one of them which happens to hold all FSMO Roles goes down.

Please correct me if I'm wrong but from what I understand:

No Schema Master = no updating user details, no creating new users\computers\groups\group policies.
No RIF Master = no applying altering or adding new permissions to files\users\groups.
No Domain Naming Master = no adding new domains to the forest.
No PDC Emulator = no time synchronization, no password change or reset, no account lockout.
No Infrastructure Master = no cross-referencing objects between domains but only where you have a non-global catalog DC. Also no deletion of objects takes place.

Also, let's assume that the DC that contained all FSMO roles went dark for 1 hour and came back online. I suppose that the changes that were performed during that time will be will be overwritten?

You literally need to read the documentation. It tells how things are handled and will handle a lot of confusions.

I.e.:

No Schema Master - you are WRONG. The schema master is responsible for the SCHEMA. Updating user details is not changing the schema. Schema updates are (i.e. adding a field to the user object).

No RIF master (it is RID, not RIF) - you can add permissions as you want, as this does not create a RID. The RID master is responsible for handing out RID's. Like when a USER IS CREATED (new object). Here is the point though - the DC's all CACHE UNUSED RID. So, a RID master going down for an hour is mostly a non issue unless you try to create thousands of users on another DC during this time.

No PDC Emulator: Time Sync not sure (as this is done via PDC emulator), but why PW change or reset? YOu have so old windows machines that they use the old PDC concept / API? Because otherwise there is no reason for this. I find info saying otherwise in not necessarily current technical info so - a little complex.

The list goes on. YOu seem to make up limitations because it looks like you have no real idea what the roles do.

Anyhow, the exact documentation you look for is here:

https://docs.microsoft.com/en-us/troubleshoot/windows-server/identity/fsmo-roles

Generally assume that roles are not necessary to be operational all the time - in many cases the system will work with limitations (i.e. no update that requires a schema update when the schema master is offline, but you CAN update OBJECTS as this is not a schema update).