There is by no means some simple answer to this, nor a simple solution.
You do not say what hardware you are using. This could make a difference to how solutions are presented. Apple's mobile devices can only accommodate one account (for most purposes) at a time. Computers can accommodate multiple Apple Accounts when set up as separate users.
The extent to which data remains accessible or can be shared depends upon how those data can be accessed and where they are stored. If data are Apple account specific then you have to be signed into that account in order to get access to them. For example, health data, I believe, require you to be signed in with the applicable account. Some data may require you to be signed in with a third party account and do not depend upon the Apple account being used.
When you sign out of one account on a mobile device you are presented with a list of options for retaining data or deleting it. If you keep this data when signing out of one account and then sign in with a different one, if the data are the kind also stored in iCloud then you will find the data are merged. This can get messy if done multiple times. I believe some data such as passwords are not retained, however. Data such as Contacts will merge though.
Note that for the purposes of iTunes and App Store purchases you would also run into:
View and remove your devices that are associated with your Apple Account for purchases - Apple Support
‘When you try to associate a device that was previously associated, you might notice a message that says, “This device is associated with another Apple Account. You cannot associate this device with another Apple Account for [number] days.” You might need to wait up to 90 days to associate the device with a different Apple Account.’
If you download a Thai app or a purchased (i.e., not Apple Music subscription) Thai song and then sign out of that account telling the device to keep data, and sign into the USA store, you will still be able to use that app or play that song even though you are signed into the USA store. You will not, however, be able to update that app or re-download the song should the need arise.
As mentioned in another reply, I would avoid doing anything which Apple could possibly deem as account abuse. According to various posts, Apple may permanently lock accounts for other reasons (very broadly outlined in section VII-B of Legal - iCloud - Apple ). Apple does not recognize dual country residency and when you change country you should change the settings on your single account and that's it. The support article Change your Apple Account country or region - Apple Support describes how to change your country setting on everything using your Apple Account, and the requirements and results of doing this.
If you truly wish to maintain accounts in both countries (and can surmount challenges in Apple's system enabling you to do this) then you are best off using separate accounts on different mobile device hardware (or different computer accounts). Keep things completely separate.
