0:39 The basics (about legacies, item xp, forge-master, basic approach to legendary items)
16:02 Advanced options (about relics, relic-master functions, legacy replacement, legendary titles, scroll of empowerment, delving, star-lit crystal, crystal of remembrance)
41:30 Imbued legendary items (implications, approach, functions of scroolls and crystals post-imbuement)
The basics are important to understand from the beginning, along with the tip given at 23:26. The Advanced options become important towards high levels. Imbuement is only relevant from level 100-105.
Every legacy per class, regular & imbued: