From what I understood Porsche and Audi would enter as an engine manufacturer, not a complete new team.
(Porsche teaming up with RB, and Audi looking to buy mclaren or maybe another team).
And that is what usually happens. Mercedes wasn't a new team when entering. They bought Brawn GP, which was Tyrell before that (and some other names I think)
New teams entering F1 doesn't happen often (a new team that builds a chassis I mean), it's usually by purchasing another old team.
Red bull was jaguar racing, which was steward gp
Alpha tauri was minardi, which was some other names
Alfa romeo is sauber
Haas was marussia
etc
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