In Android, there are several methods in which a folder can be selected. Two method are available in the app:
The original method, where the app just has access to the file system, and
The new method, where the user provides the app access to a specific folder via a system window
The app defaults to the original method, but the user can switch over at any time. Once a folder is selected by either method, the app behaves exactly the same.
The best method will vary by device - if you are unsure just try both methods to see if you can find the appropriate folder.
In general, if you are using an external device such as an SD card, thumb drive, or hard drive, the newer method will be better - and quite often will be the only method that will work.
If you are using Internal Storage, the original method will usually be easier.
By default, the app uses the original method. When you select "Choose Folder" or "Change Folder" on the main screen, it will show a popup at the current folder, or the default storage folder. From here you can go down into sub-folders or back up to a parent folder. When you're in the right folder (that is, it lists the albums you want to upload), just "Select" the folder, and it will start scanning.
If you've switched over to the new method, you can switch back in the app settings, under "Folder Selection"
If you need the new method, you can turn it on in the app settings, or you can get to it from the folder selection popup ("Help" > "Try It"). Once enabled, "Choose Folder" or "Change Folder" will start a selection using the new method.
A new window will open. The menu will usually show the Downloads folder, and any enabled external devices by default. You can select these, or any sub-folders within them. Once you have found the right folder (you can see the albums you want to upload), just "Select" the folder and it will start scanning.
You may need to do a little more to find the right device or folder - see below.
External devices are usually accessed via "OTG" (On-the-go). On some devices, such as OnePlus, this is always off by default. When you connect an external device, you will need to manually switch it on (this might be done through quick settings in the notification area, or through the settings), which will enabled it for a short time. As long as the device stays connected, it will then continue to be available and should appear in the system folder selection window (even if it's already open).
By default on most devices, only the Downloads folder will appear when using the new method. You can enable it using the menu and selecting "Show Internal Storage". The main menu should then show the whole Internal Storage.
The methods of accessing folder in Android 10 and going forward will be more limited, but the new method should still work to find all the files you need. An update will be provided in the near future to clarify, and the new method may be used as the default or only method going forward.