For very concise introductions, check out the 1000-word essays on philosophical topics written by philosophy professors. https://1000wordphilosophy.com
For a deeper dive also written by philosophy professors, the Stanford Encyclopedia of Philosophy is unrivaled. https://plato.stanford.edu
For a deeper dive also written by philosophy professors, the Stanford Encyclopedia of Philosophy is unrivaled. https://plato.stanford.edu
Richard Marshall's 3:16am interviews with philosophers are also great. https://www.3-16am.co.uk