I'm new to the field of investment and would like to understand ETFs, they have not been clear to me even though I tried a bunch of YouTube tutorials. Also, how is it possible for a European resident to trade U.S. ETFs?
An ETF is essentially just a package of funds. It can include stocks, bonds, or other commodities. ETF shares can then be purchased in the same manner that you would purchase individual stocks. This is similar to a mutual fund. The main difference between a mutual fund and an ETF is that you need to purchase individual shares/units of an ETF, whereas mutual funds can be purchased in a continuous manner. e.g. If you have $500 to invest and an ETF costs $120 you can purchase 4 units of the ETF and have $20 left over. Comparatively with a mutual fund you can buy $500 worth of mutual funds. Some people may choose ETFs over mutual funds if their banks or brokers don't allow for the purchase of specific mutual funds.
I would suggest reading JLCollins's Stock Series, which will give you a primer on stocks, bonds, ETFs and Mutual Funds. It's mostly focused on index funds, but you'll gain a better understanding of how everything works.
For information on investing in US equities as a European resident I'd refer you to these questions: