If you want to ensure your exploration is well rounded, I usually recommend the book called 1,001 Movies You Must Watch Before You Die, or the list of 1,000 Greatest Movies curated by They Shoot Pictures, Don't They? (which gets its name from the great movie They Shoot Horses, Don't They?)
https://www.amazon.com/1001-Movies-You-Must-Before/dp/1438050755/ref=sr_1_1
https://www.theyshootpictures.com/gf1000.htm
I like these lists because they include a short blurb about each title that succinctly point out why they made the list.
Just watch lots and lots of movies. I usually recommend this book as a good list of essential viewing.
https://www.amazon.com/1001-Movies-You-Must-Before/dp/1438050755/ref=sr_1_1
or, if you prefer an online list, this one is very similar, as it is created by amalgamating titles in that book with several similar lists from various reputed sources...
https://www.theyshootpictures.com/gf1000.htm
Those are both good lists because they include a healthy sampling of world cinema as well as US cinema.
Pick a movie from the list, watch the movie and then read the write-up. I also like to read reviews by prominent critics right after watching, as well as scanning the movie's wikipedia page. I prefer to read about the movie AFTER watching it so I can form my own pesonal impressions first.
"Top" lists from CineFix are usually very informative, as they tend to include examples all through film history as well as including non-American mentions.
https://www.youtube.com/c/CineFix/search?query=top%20
Cinema Cartography is usually very educational.
https://www.youtube.com/c/TheCinemaCartography/videos
The Discarded Image has a few excellent essays.
https://www.youtube.com/c/TheDiscardedImageChannel/videos
FilmmakerIQ's series of vids from about 7 years ago taught me a lots about subjects like the history of editing, the history of copyright in movies, the history of title sequences. Just scroll down to a series with similar looking thumbnails featuring posed figurines and titles that often being with "The History of..."
https://www.youtube.com/c/FilmmakerIQ/videos
The Royal Ocean Film Society makes an interesting mix of videos about various film related subjects...
https://www.youtube.com/c/TheRoyalOceanFilmSociety/videos
... as does Thomas Flight ...
https://www.youtube.com/c/ThomasFlight/videos
Studio Binder is aimed a makers, but is also very educational from a viewing perspective
https://www.youtube.com/c/StudioBinder/videos
If you are looking for watchlists, I recommend either the book called "1,001 Movies You Must See Before You Die"
https://www.amazon.com/1001-Movies-You-Must-Before/dp/1438050755/ref=sr_1_1
... or the very similar list maintained on the website called They Shoot Pictures, Don't They?
http://www.theyshootpictures.com/gf1000.htm
https://www.theyshootpictures.com/gf1000_all1000films_table.php
My favourite source for many years has been the book 1.001 Movies You Must See Before You Die
https://www.amazon.com/1001-Movies-You-Must-Before/dp/1438050755/ref=sr_1_1
If you prefer not to use a book, this website also has a very similar top 1,000 list, which is compiled from various sources.
http://www.theyshootpictures.com/gf1000.htm
These lists are great because they include a nice cross-section of world cinema. They also feature short write-ups for each title that give a little background, and help you understand why they made the list.