The backdoors the NSA uses are known vulnerabilities, 0days, USB drops, all the normal hacker tools… and if it’s a target of sensitive enough in nature, maybe a warrant requiring Apple Update/Windows Update/whoever or whatever device needs spied on, to deliver a payload to that specific machine.

https://www.vice.com/en/article/wnx5bm/nsas-hacker-in-chief-we-dont-need-zero-days-to-get-inside-your-network-rob-boyce