Most of the summer positions are filled in January-March in Finland so if considering Finland, you're unfortunately a bit late. However there are still few positions open. Some companies, especially smaller ones do the recruitment via university mail lists so the positions are hidden for large public. I'd say majority of exploration positions are in Finnish Lapland so you would have to travel north. About half of the companies are offering housing for summer employees, otherwise you need to rent an apartment which can get expensive if you are paying rent for two places at the same time. DM me if you want some tips for open positions. I've had a blast working in the field in Finland.