How does Firebase choose what to store in its cache with isPersistenceEnabled = true in iOS

I have an app that is using Firebase quite extensively to store data that contains relationships. I want to make sure I am using Firebase as safely as possible in offline mode. The safety concern I have can be demonstrated in the following example:

Assume I have a Zoo model where each individual zoo is stored in Firebase as a subnode of "/zoos".

I have an Animal model where each individual animal is stored in Firebase as a subnode of "/animals".

A Zoo can have Animals which are stored in an ordered list. Specifically, the Zoo model contains an Animal array e.g. [Animal]. This list of Animals is stored in Firebase as a set of position-reference pairs at "/zoos/myZoo/animals" which will contain nodes like:
0: "animals/fidoTheDog",
1: "animals/jillTheCat"

When I add a new Animal to a Zoo, I need to know how many animals are currently in that zoo so I can add the new animal in the right position like:
2: "animals/jakeTheSnake"

If I am offline and happen to read the location "zoos/myZoo/animals" to get the list of animals so I can add in the right position, I want to make sure I have accurate data. I know that if someone else wrote to that position while I am offline and added another animal in position 2, I will get stale data and when I add an animal in position 2, I will overwrite his entry at "zoos/myZoo/animals/2" when I again go online. So that is an issue.

But, if I know I will be the only one writing to that location, can I be relatively sure that Firebase will hold the crucial data at "zoos/myZoo/animals" for me since I am using isPersistenceEnabled = true? In other words, will Firebase just keep that data in cache as long as I have recently written to that location or recently read from that location?

Or do I explicitly need to specify "keepSynced(true)" on that location? This gets to the core general version of the question - How does Firebase choose what to store in its cache with isPersistenceEnabled = true? Especially if I have not specifically set keepSynced(true) on any particular locations. Will Firebase just prioritize recently read data and then when the 10mb limit is hit, discard the old stuff first? Does it matter if I wrote the data to that location a long time ago but consistently read from that location? Will it still maintain that location in the cache because it was recently read? Will it ever discard data before hitting the 10mb limit?

I'm a little bit of a newbie so thank you for your patience with me!

-------------- FOLLOW UP QUESTIONS --------------

A couple follow up questions.

I think the approach suggested in the blog (given by Frank in comments) of using childByAutoID sounds good. So if I am saving a zoo with many animals (in order) then it sounds like I would loop through the animals and use childByAutoID to create a new key for each animal whose value will be the reference to the location of the animal object. Can I be sure that the keys that I create in rapid succession (looping will probably be very fast) will ultimately sort correctly when ordered lexicographically? I’m looking at this blog post and assuming that is the case. https://firebase.googleblog.com/2015/02/the-2120-ways-to-ensure-unique_68.html

Suppose I am doing something more complicated like inserting an animal at the beginning of the list in position zero. Then before doing the operation, I would sync down the list of animals in the zoo as suggested in the blog post you sent. https://firebase.googleblog.com/2014/04/best-practices-arrays-in-firebase.html. If the user is offline, I obviously can’t be sure that I will have the freshest copy. But suppose I am ok with that because users will only be working with their own data and only on their own device. In that case, does it help to use keepSynced(true) on the path to the zoo? Or since the amount of data the user is working with is well, well under 10mb (the whole database right now is 300k for 10ish active users), can I just assume the cache will store the data in the zoo path (whether keepSynced or not) because we never flirt with the 10mb limit in any case?

Thank you!

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