I'm working with one feature class with a lot of redundant fields, but I want to make the person inputting the data's job a bit easier. So if field A isn't blank, I want field B to update to "Yes". Depending on the answer to field A, field C would get answered in one of two ways. And so on. This would be updating pre-existing records that already have a value, there is no data creation.
To clarify, do you want this behaviour to occur while a user is editing their records' attributes - for example, after they set field A, field C's value/behaviour changes - or you want a process to come in after the user has completed their work and calculate field values based on what they entered?
If it's the first case, it's complicated because there's no way to enforce that kind of behaviour at the data store level in ArcGIS Online. You need to ensure your users are using clients that support that kind of thing, either Arcade-enabled forms through something like Field Maps or the Web Editor, or something like Survey123 that supports conditional logic in its own way. Make/Integromat/etc. can apply edits but it can't do things like change field visibility as Field Maps/S123/etc. can do, or limit selections to subsets the way Survey123 can do.
If it's the second case, I find scheduled notebooks are the best way to accomplish this. I'll often include a "processed" field with a flag of 0 or 1, if the notebook has processed the record to apply the calculations it flips the value to 1. This lets me hide records pending processing to users, as well as avoid processing records more than once. An exception to this would be if your data needs to be updated in near-real time, in that case Make/Integromat/etc. are your best bet since notebooks can be scheduled to run at a maximum of 15 minute intervals.
