That did he write his thesis on, The Giver?

ext4 would be normal for linux and windows is usually formatted as ntfs, there’s nothing abnormal with the partitions if you were trying to do just mint, unless the install messed with something weird in the bios I can’t imagine why it wouldn’t boot if you erased the drive while installing mint only, the installation should select that partition to boot without you having to do anything extra

Can you load in to the live usb for mint and see what it looks like when you open gparted? When you reinstalled did you select erase the whole drive or another option?

Here are two videos on clean installing either ubuntu and windows just in case.

https://www.youtube.com/watch?v=bP03Y-l9NOM

https://www.youtube.com/watch?v=dOoyem2xOyc

Even if you want to keep on dual booting you’ll have to get one installed first anyways.

often times if you had backed up any important files before doing this you are gonna waste less time by just doing a clean reinstall, but you can definitely try to troubleshoot fixing the boot