While redwood is naturally resistant to rot, it will grey and split if left unprotected. Most furniture makers I know suggest either Danish Oil or Polyurethane.
You’ll hear a lot from people trying to sell you their ‘special’ formulation telling you to not use varnish or polyurethane but to only use their product, but I’d ignore them.
If you use danish oil, you’ll need to reapply it regularly, at least annually, but I do love it as it dries hard and nourishes the wood. Varnish or polyurethane is lower maintenance.
I’ll add a couple of links for you to look at, the Rustins product is a good one as it’s clear but has UV blockers, the Danish oil has varnish added, and the Spar Varnish is great for exterior furniture.
Links to Amazon: https://geni.us/IozciS9 and https://geni.us/B1WvHH
I hope that helps.