Digging straight down is a speedrun strategy of last resort for modern versions. The typical progression to enter the Nether is to start by getting 7 iron (3 for an iron pick, 3 for a bucket, 1 for a flint and steel). You want to get that, grab some food, and be in the nether at around 3 minutes.
Doing that by digging down is not really feasible. There are two main routes.
In the village route, you spawn next do a village with a blacksmith. You get some iron or an iron pick from the blacksmith, kill the iron golem for the remaining iron, grab some wheat and then find a nearby lava pool or completable ruined portal. There are variations where you can trade with a villager for a bucket, but by then most speedrunners would have already reset.
The second and more typical modern route is using ocean. You locate a shipwreck which gives you iron and food. Then you find a ravine that has magma blocks (indicating there's lava underneath) and use that to build your portal. To my knowledge this isn't viable in 1.18's world gen though.
When speedrunners do go underground in the overworld, they'll usually try to find a cave first to find exposed iron and lava. Digging down is the last choice.
Doing that by digging down is not really feasible. There are two main routes.
In the village route, you spawn next do a village with a blacksmith. You get some iron or an iron pick from the blacksmith, kill the iron golem for the remaining iron, grab some wheat and then find a nearby lava pool or completable ruined portal. There are variations where you can trade with a villager for a bucket, but by then most speedrunners would have already reset.
The second and more typical modern route is using ocean. You locate a shipwreck which gives you iron and food. Then you find a ravine that has magma blocks (indicating there's lava underneath) and use that to build your portal. To my knowledge this isn't viable in 1.18's world gen though.
When speedrunners do go underground in the overworld, they'll usually try to find a cave first to find exposed iron and lava. Digging down is the last choice.