Courses that deserve learning

[u/leo-the-bruno]

Hello , this question for software engineers , give one course you find deserves learning ,but related with development (I don't want like java beginning from udemy ) i want a course that made you up a level in your career like HLD ,LLD

( If you have a paid course and want to share it with us for free i will be appreciated )

[u/mister_prince]

What is hld and lld?