Yaarel
🇮🇱 🇺🇦 He-Mage
"Custom lineage" means creating a new lineage that isnt elf/dwarf/etcetera.Both are wrong. In 5e so far, Lineages are things that you can be born as (like a race), but also something you can become. Custom Lineage, Dhampir, Reborn, and Hexblood are all things that you can become. Aarakocra, Tortles, Gnomes, and so on are all races, because they're all something that you're born as, while a Lineage is more a "mutation" that you can either be born with or get later in life.
Race is still used, as shown in The Wild Beyond the Witchlight's Fairy and Harengon races.
Each lineage (including its player character races) is assumed to have kin, meaning there are others like this new lineage.
Technically, elf/dwarf/etcetera are "lineages", while only their player character aspects are referred to "races".
Lineage/species = PC race + NPC/monster
But a lineage (including player race) might be something one can become. For example, there is a draconic option for a player character to become a dragonborn.
Likewise lineage is something one can create, like the original dragonborn that dragons created by transforming unhatched dragon eggs. Presumably warforged reproduce by learning how to build and create new warforged.