Hi Robinson! I would say that deem is very formal and rarely used in everyday language in comparison with think. Think can be used for general thoughts, but deem is used specifically for opinions or perceptions, and tends to add a sense of authority to your judgement, especially in legal contexts. When I think of the word deemed I usually think of it in a set phrase such as 'deemed worthy' or 'deemed obsolete'.
I hope this helps clear up the differences for you. Send me a message if there's anything else I can help with :)