We made a tool that lets you search the lichess game database by chess concepts. Link in the comments : r/chess

How to Win at Chess Openings - Complete Guide - TheChessWorld

Lichess Database: Which openings are most successful : r/chess

learning - Where can I find free training tools like detailed annotated chess openings, middle games and endgames in the PGN format? - Chess Stack Exchange

How do I switch from Masters database to Player database for opening explorer? - Chess Forums

what do the numbers for each variation in chess opening mean by? - Chess Forums

f4 - Chess Opening Database

Opening Encyclopaedia 2023

Chess Repertoire Explorer

What is the single best chess opening for average players to master? - Quora

Quantifying the complexity and similarity of chess openings using online chess community data