Website visitors, sessions and session length have dropped significantly. How do I locate the problem?

Hi guys,

I am noticing a downwards trend in my weekly visitors. It started around Wednesday the 15th of August. Both visitors, session length and sessions length have decreased by about 20% since then.

Besides reading data, I don't have much knowledge about Google Search Console / Google Analytics, so I wouldn't know how to locate the problem.

Any tips?