Hi and welcome.
Our summerhouse I built in 1987. It's made from roofing ply and soft wood. It's had many coats of Dulux Woodsheen, more recently called "Trade Woodstain." It forms an inpenetrable surface around the wood.
The first coat was straight onto bare wood.
This comes in other colours but it isn't cheap, around £50 for 2.5 ltrs, it goes further if the wood is smooth.
The bulding sits on low brick piers on paving slabs with a scrap of pondliner on top of each pier. It has side "skirts" attached to the floor that finish a couple of inches above the path.
There's absolutely no rot anywhere.
Our shed I made in a similar fashion again on piers. But from reclaimed softwood, roofing ply and window frames,
I built it to house our daughter's rabbits she had at the time, when she was seven.
She'll be 53 this year, (there's no rabbits in it now and she left home decades ago!)
Still no rot,