When you format a microSD card to be 'internal' (a.k.a. 'adoptable'), the card is changed in two very distinct ways. It' s file system is changed from FAT32 or exFAT to ext4 (the same file system that your phone's internal storage uses), and it gets encrypted (with the encryption key residing in your phone's internal storage at a system-level) The card at that point is now considered to be an integral part of your phone's internal storage so don't bother using at as a file transfer media any longer. The Android OS running on your phone considers it to be just part of the overall storage media, and because the encryption key specifically ties the card only to your phone it won't be mountable or usable on your computer/laptop anyway Also, neither Windows nor Mac OS include support for ext4 file systems anyway. Well out of the box, unless you install third-party utilities to do so. Basically, a microSD card formatted as 'internal' has to remain in the phone all the time, and it's of no use on a separate computer, or other phone, anyway -- the file system is non-native to most computers (Win and Mac) and the encryption isolates to the card from any other computer device.
If you later decide you want to change your card back to being 'portable', you must do this from within your phone. Once a card is set up and used as 'internal', it's the Android OS that manages what does and doesn't get written and resides on the card media, you (the user) don't really interact with the card by manually copying files to or from it. So changing it back to 'portable' isn't just a matter of copying all the individual files off of it, there are likely to be various app contents that will be disrupted. Again, as far as the Android OS is concerned that card was merged into your overall storage media so the contents of the card will likely consist of a mix of files, app data, and other system-level files. By changing your mind on the card's usage, this is something of a big deal. So also again, do this on your phone because it has to account for the apps data and config files too. Don't be surprised about losing data or having to reinstall some apps when doing this. That's just another reason why backups are so vital, backups are important.
But once you do format your card back to 'portable', that returns it back FAT and thereby removes the encryption so you can use the card as a transfer media again between your phone and your computers/laptops.
Keep in mind that FAT32 and exFAT are both just derivatives of Microsoft's FAT file system. FAT is very dated and none of its derivatives are actively supported and developed so it has serious drawbacks with today's technology. (It's also proprietary and Microsoft stopped supporting it years ago.) FAT was only a weak, unstable file system even in it's heyday (Win 98 and WinXP) and it remains that way. It's only plus point is Microsoft allows other platforms to support it so that's why it remains as the default file system for USB storage media -- when we (all consumers) buy something like a SD card, you can use it as is in a Windows, a Linux, or a Mac computer. But as you've pointed out (file size being one of them), FAT does have a lot of limitations that have only increased in number as time goes on. Blame the USB-IF board for the curse of USB storage frailties. It's not just a matter where they drank the Microsoft Kool-Aid and picked FAT to be an industry default way back, but they continue to do so even decades later.
https://en.wikipedia.org/wiki/USB_Implementers_Forum
So make your decision on how you want your microSD card to be set up before-hand. If 'internal' just stop thinking about using it as a transfer media and don't bother trying to manually interact with it on your phone; if 'portable' you'll probably get better results formatting it to use exFAT since you're anticipating the need to support 4GB or larger files.
