now i'm confused about why solargraph, which is said to be mature, can't do that

It's important to note that ruby-lsp is made by Shopify, and is currently used in-house by the company. The resources they can invest into ruby-lsp both in terms of development man-power and project management is naturally going to be higher than the community developed solargraph.

and also i just realized it when i used map instead of select; the methods appeared.

That's strange, but Enumerable#map and Enumerable#select have different uses. One is used to execute a block across a collection of elements, while the other constructs a new collection based on the block passed to a collection and the results of evaluating that block against each element.

wanna try RubyMine but i already comfortable in neovim. since RubyMine is an IDE i think my machine can't handle that lol.

I use RubyMine on my desktop and it's a very solid development experience. I'll say that if your machine can't quite handle RubyMine, VS Code(ium) is a nice alternative with the solargraph or ruby-lsp plugins. I don't know if they'll have the same completion issue as neovim though.