This channel I've found very helpful https://youtube.com/c/TranscendFurnitureGallery More on reddit.com

Depending on the condition of the wood, I may want to stain/varnish, or paint (likely a lot easier I would guess). After this, there is a desk and small dresser as possible projects. I am becoming increasingly interested in furniture refinishing/restoration as a hobby. More on bogleheads.org

Teak oil is used to protect wood that is left outside. Sand the furniture by hand using 120 grit paper, wipe or wash off the debris from sanding, wipe on teak oil, let it dry 24 hours, wipe on a 2nd coat of teak oil. More on reddit.com

This needs need a coat of water sealant. If those pieces come apart then take apart one chair at a time, mark the undersides so you know how to put it back together, and seal every surface. Although sealer costs a little more than paint, it's frugla in the long run by extending the life of your patio furniture. Be sure to prep properly before sealing. Give it a good cleaning and sanding. If parts are badly warped or split then you may need to buy lumber and replace those parts. Replacement lumber won't match the shade of the existing wood, so if you're replacing parts then select an opaque sealer. Opaque sealer looks similar to paint and is sold in the paint section. Pro tip for future reference when you eventually need to replace this: budget conscious wooden patio furniture may be sold unfinished. We noticed this in the fine print when bought a patio set from IKEA, so we headed to a home improvement store for sealant and treated the whole set before the first rain. That's still frugal compared to the sticker price of higher end furniture, and seven years later it still looks good. More on reddit.com