China build artificial islands in South China Sea

US tells China to remove missile systems in South China Sea
It'll have the same effect if China tells the US to close its bases on Okinawa, Guam & Diego Garcia, Japan tells Russia to do the same on S. Kurils, or if Argentina tells the UK to demilitarize the Falklands.
https://svpressa.ru/war21/article/215727/?utm_referrer=https%3A%2F%2Fzen.yandex.com

And there too: In the Maldives, in the Indian Ocean, rising sea levels are causing salt water to intrude into underground fresh water supplies. In order to adapt, the country is trying to build rainwater cisterns and new pipe systems to ensure that its people have safe drinking water supplies. https://www.nytimes.com/2017/11/17/climate/islands-climate-change-un-bonn.html

US drops the gauntlet on the South China Sea ..China’s deployment of surface-to-air missiles and anti-cruise missile systems to contested islands earlier this year..- note that those r defensive systems! If they put AshMs there, they'll be used in defense to break the blockade or invasion.

War: US will not stand more than seven days against China: https://regnum.ru/news/polit/2562064.html

The JSDF won't be of much help; SK has a territorial dispute with Japan, so both Koreas r on the PRC's side.
China eyes a strategic port in the Philippines
Chinese companies look to win control of a bankrupt but strategic shipyard at Subic Bay, the ex-site of US military bases and a potential key outpost on the South China Sea

https://thediplomat.com/2017/06/china-deploys-advanced-anti-submarine-warfare-aircraft-to-south-china-sea-air-base/

https://www.janes.com/article/89352/canada-japan-navies-enhance-interoperability-in-south-china-sea

Europe is expanding its naval presence in the contested sea, adding legitimacy to US free navigation operations and affirming China’s anxieties of Western encirclement
https://www.asiatimes.com/2019/06/article/coalition-of-the-willing-builds-in-south-china-sea/

Messing with the PRC SLOCs around SE Asia won't be easy now.
This koan is applicable here: Who can remove a bell from a tiger's neck? A: only the 1 who put it there.

5 changes the Navy and Marine Corps can make to take on China

I don't think it'll help them much: even if those SC Sea islands r taken, China can bombard them with BMs until they leave & then retake them.

Duterte delivers deathblow to US-Philippine ties by scrapping key bilateral security pact that US leverages to check China in SC Sea

https://nationalinterest.org/blog/buzz/us-philippines-alliance-dying-123841

https://www.scmp.com/week-asia/politics/article/3050833/ending-philippines-us-military-pact-will-affect-south-china-sea

https://www.nytimes.com/2020/02/11/world/asia/philippines-united-states-duterte.html

I suspect that he plans to invite Russia to counterbalance China, if need be. Then it will be a more stable RF-Philippines-PRC triangle than previous Japan-Philippines-USA-Australia quad.

https://nypost.com/2020/04/08/us-blasts-china-for-unlawful-claims-in-south-china-sea/

https://www.nytimes.com/aponline/2020/04/06/world/asia/ap-as-south-china-sea-watch.html

https://thediplomat.com/2020/04/south-china-sea-whats-behind-china-vietnam-tensions-amid-covid-19/

https://learningenglish.voanews.com/a/natural-gas-discoveries-could-add-to-chinese-claims-in-south-china-sea/5352490.html

Well, the Indochina peninsula isn't owned by India or China. Okino Tori Shima in the Philippine Sea belongs to Japan. https://en.wikipedia.org/wiki/Okinotorishima

Likewise, only a few offshore islands+Nicobars in the Indian Ocean belong to India. So, geographical names alone can't be used to claim anything.
Indeed, the US has a double standard- 1 they apply to themselves & the other to rest of the world- a superpower must have "super rights & privileges". After all, $ is the World's reserve currency & the UN is in NYC. They possess Guam, Hawaii, Alaska & lease bases in Greenland, Japan, Diego Garcia & Singapore to help police the oceans & bomb enemies.

How can China not own islands it built itself?

The US & others say they were reefs & shoals in disputed waters & China built them up illegally. From PRC's perspective, they belonged to them for a long time & her geoeconomic/political status entitles her to secure the SC Sea within the 9-dash line- a maritime equivalent of the Great Wall. With AD/AD there, it would be harder to aid Taiwan & Vietnam, not to mention to bomb the S. Chinese coast.
Also their SSBN bastion is there- turning that sea to a strategic asset.

Update: https://asiatimes.com/2020/04/china-lays-ever-larger-claim-to-south-china-sea/?utm_source=The+Daily+Report&utm_campaign=fdb7f68855-EMAIL_CAMPAIGN_2020_04_21_11_29&utm_medium=email&utm_term=0_1f8bca137f-fdb7f68855-31607385&mc_cid=fdb7f68855&mc_eid=5455568640

https://asiatimes.com/2020/04/china-could-soon-sink-us-in-south-china-sea/?utm_source=The+Daily+Report&utm_campaign=e8a1a4b469-EMAIL_CAMPAIGN_2020_04_23_10_35&utm_medium=email&utm_term=0_1f8bca137f-e8a1a4b469-31607385&mc_cid=e8a1a4b469&mc_eid=5455568640

Are these artificial islands concrete or sand based?

Probably corral sand &/ rocks.

https://www.express.co.uk/news/world/1285288/South-china-sea-news-paracel-islands-Beijing-navy-crops-vegetables

https://recordnews.uk/2020/06/04/south-china-sea-latest-beijings-plot-to-claim-contested-region-revealed-world-news/

https://www.gazeta.ru/army/2020/07/11/13148959.shtml

https://www.forbes.com/sites/hisutton/2020/07/17/china-sends-flanker-fighter-jets-to-south-china-sea-amid-tensions/#3c2f7e302f73

They may deploy more fighters if need be-this may be an advance party.

Mike Pompeo, the US secretary of state, said last week that the world would “not allow Beijing to treat the South China Sea as its maritime empire” and that the US would “stand with the international community in defence of freedom of the seas and respect for sovereignty” in the region. https://www.theguardian.com/world/2020/jul/23/south-china-sea-australian-warships-encounter-chinese-navy-in-disputed-waters

The US had the Philippines & still has Guam while maintaining defense ties with Taiwan next to the SC Sea, as well as the Caribbean Sea "maritime empire" around Puerto Rico & Panama it dominates. https://asiatimes.com/2020/07/china-bids-to-win-back-the-philippines/?mc_cid=ed3dfe2acf&mc_eid=5455568640

Hawaii & Diego Garcia allows them to have "maritime empires" in the middle of the Pacific & Indian Oceans.