Over the last two days built a new home for the scooters....
I am to the point I need to put up the soffit and fascia and was hoping for some advise.
I have open rafters with a 12" overhang. Was hoping to use vinyl soffit with alumn. fascia(vynel soffit for simplicity of cutting and $).
All the instructions I see online suggest putting F-channel on inside wall facing out, and the outside trim board facing in. I have in the past used the fascia to hold up the soffit.
Have I been doing it wrong all these years? Or is there a special reason ya need to do this with vinyl?

Thank you in advance.

Thats right. Just don't install the vinyl soffit tight between thechannel and the fascia too tight. Vinyl expands and contracts a great deal. It will end up looking wavy. Do you have a ceiling in your garage/shed? If so, you should use a perforated soffit to vent the eave and a ridge vent (assuming your using shingles)on the roof. Attic ventilation is very important to prevent drying out of the sheathing.

The simple reason for using vinyl over aluminum is cost. Aluminum would still need to be coated with a finish, usually PVC.

They did it just the way you describe on our house, the fascia is holding the soffit. Well not exactly, they did put a few nails in the soffit but there is no J channel on the fascia for the soffit.

Is it right?

That's questionable but I guess it saves putting up a bunch of J channel.

I have always placed 1 small trim nail in each piece of soffit into the fascia board,from the bottom, this way you don't need so many nails in your alum. fascia. Plus when you put up the Alum. fascia.....nail it from the bottoms side through the 1" return up into the vinal soffit and into the wooden fascia, thus eliminating face nailing the Alum. fascia (except at the joints)....hope this helps!