Use external storage as primary default storage

First, I do not want to boot from an external drive, only store data on one. I want to get that out of the way first since every discussion on my query tells me how to use an external drive as the boot drive. I don't need that.

Now that Apple is welding storage into their machines, I will not be able to expand storage when my machine gets full. Also, decades of experience shows me that drives do fail, whether HDD or SSD. They just like to go poof after awhile. Even in Macs. I want the freedom to swap out drives when I need more room or one burns out. Just like it was in the old days.

I want to buy a new M-chip series Mac, but I don't want to buy 2 gigs storage right now because I don't know if I will fill 2 gigs within 5-7 years, the average lifespan of these machines. But I also don't want to find myself short on storage in that time because increased OS and app complexity takes up room. Also, I simply expect a drive to go poof in that time and I don't want to have to go through the time and trouble to replace an entire machine cause the storage drive fried. I want to fix things now, and on my own, like it was in the old days.

How do I command a Mac to always use an external storage drive to be the primary storage device? What keys do I click in MacOS to make this happen? Is this something done in Disk Utility?

Note, I'm not asking about Time Machine. That's a different beast. To use old Windows parlance, I want an external drive to be the Mac's a: drive. How do I set this up?

I apologize for the redundancy in this question, it's just that everybody and every site tells me how to boot from an external drive. I can do that with an onboard 256 gig drive. I want everything that I normally store on an internal drive to go to an external drive by default. What are the steps to do this?

Thanks!