As far as I know, this is not super well supported in vanilla VS Code (I.e. with no extensions) at the time of this writing.
There is the View: Split Editor in Group command which was introduced in version 1.61, which you can access from the command palette, and is bound by default to ctrl/cmd+k,ctrl/cmd+shift+\, and also accessible from the context menu of the tab handle. It seems to only allow splitting horizontally, and only split the same text document.
There's a quite popular feature-request issue ticket covering what you're looking for (if I understand correctly): Allow to open two distinct editors side by side in one group #36700. You can give it a thumbs up reaction to show support for it and subscribe to it to get notified about discussion and progress. Please refrain from making noisy comments like "+1" / "bump".
In that discussion thread, someone wrote an extension that they claim provides a solution, which perhaps you could try out: agquick.workviews (I have no affiliation with that extension).
There's also a similar feature-request here that you might be interested in doing the same for: Tabs that group editor grid layouts (Vim-style tabs) #143024.
For your reference / learning purposes, I found the above issue tickets by googling "github vscode issues split inside tab".
