ELI5: personal vs business funds and co-mingling

[Lisa Koontz]

Background: Just starting out, and I'm trying to wrap my head around how to get set up. Strategy is to BRRRR or buy-and-hold SFH. Great personal credit (780+). Planning to start small and grow into something bigger that would eventually need to be structured as a business.

Everything I'm reading says it's a bad idea to mix personal and business finances, mostly for accounting and tax purposes. BUT...does it make sense to open up business credit cards to open up more sources of funding? For example: get conventional financing (or HELOC, private money, etc.) in my name for a purchase, and use business CCs for rehab costs (likely at 0% intro rates), then refi and pay them off. Would this give me better access to funding? (Better loans using personal credit, but paying for whatever expenses I can with the business to avoid dinging my personal credit score.) Can this be done? Do I have to form an "official" business (DBA, LLC, S-Corp, etc.) in order to get business credit? Is an FEIN number required? Do I have to file taxes as a business if I have an FEIN?