I recommend, Second Brain should based on your way of working, according to your Types of consuming or creating data/ notes.

Like someone just like to keep writing then they can use Obsidean, Roam Research, LogSeq etc. tools.

If someone likes to note / store data in structured manner then Notion, Crafts, Clickup are the tools..

More than the ready Second Brain Template / Tool, Important thing is - Habit of Building it, Habit of Keep taking the notes / Habit of keep you ideas running....

Still, you can refer these free templates of Notion to start with - https://www.notion.so/templates

Hope that helps/ answer your question.