Outlook Mac - 'On my computer' folders

I use Outlook on my Mac for work email. I have an Ionos 1and1 IMAP account with a 2GB storage limit that I often exceed thus preventing more emails from being delivered. 

I used to use a windows laptop for work (and iMac for personal) and when I reached the storage limit I would use the AutoArchiving feature to Archive older emails to the windows hard drive, freeing up space in the server. 

Last October my old iMac died and I bought a new MacBook Pro. Since this was a laptop, I switched to doing work on this machine. 

Today I realised that the 1and1 storage was full and I needed to clear some space. I tried to move some emails to the 'On my computer' folder but when I tried to drag them it wouldn't work and if I right clicked and 'move' the 'On my computer' folders are not in the options (even in choose folders).

While trying to get this to work I noticed that when clicking in to 'On my computer' I had the option to import data. I chose this and Outlook imported 46k emails going way back before 2019 and duplicated the folder structure I have set up on the 1and1 server. I have absolutely no idea where these emails have been imported from? 

My questions are, does anyone know where these emails have been imported from? Does anyone know why any I cannot drag emails from the server folders to then 'On my computer folders'? 

Many thanks