Care and maintenance of deck in the garden
From what I can see, it looks to be in pretty good shape. You want to start out by getting a deck wash from your home store and applying it according to instructions. You usually spray it on with a garden sprayer and then pressure clean it off. Stick to setting the pressure cleaner at around 1800 PSI, 2000 max. More than that and you can damage the wood. Then you have to decide whether to use a stain sealer (which I prefer) or a deck paint. Depending on your use of the deck and your climate, you'll have to do this every couple of years.
The cracks in the knots are pretty common and the previous owners couldn't to much to prevent that. If they really bug you or get worse, you can replace the boards if you can find the same type at your home store or lumber yard. The boards were screwed in place so replacing a few won't be too hard. Hopefully, they used stainless screws.
This deck should last for years and will depend on how well it's cared for. Only time will tell but mine have lasted 15 to 20 years in hot, rainy south Florida