It's possible (on other platforms) to install multiple versions of Node.js and select which one to use for a given project via nvm.
Why isn't the Gentoo net-libs/nodejs slotted, so that multiple (major, at least - e.g. latest 12.x.x, 14.x.x & 15.x.x) versions of node can be installed and selected via nvm?
See bug #580698, no maintainer and no trivial to slot the package.
Questions are guaranteed in life; Answers aren't.
"Those who would give up essential liberty to purchase a little temporary safety,
deserve neither liberty nor safety."
- Ben Franklin https://www.news.admin.ch/it/nsb?id=103968
Node stuff in general would really need _new_ dedicated developers to work on this (e.g. could have targets and packages that use them), preferably implemented in overlays first.
gentoo is just barely keeping it working/updated, jer even wanted to get rid of LTS 12.x a while ago but user requests reverted the decision.
There are also nodejs managers available binary versions of nodejs: https://github.com/tj/n
I assume they are downloaded into a directory in your home.
That's one example. There are others.