SD is a resolution (usually 480p for digital/roughly 480i for analog but video resolution terminology isn’t really that precise or standardized, especially with analog video). Composite video is always SD, unless there’s some exception I don’t know about (not counting 240p from old game consoles and computers, since 240p 60fps is kind of the same as 480i 30fps but not really and also SD analog video can also be nominally 576i ut I’ve definitely used old, consumer capture devices that work at more like 520i and yeah, analog video resolution gets a bit vague all around)
SDI is an older digital video interface protocol, kind of the AES/EBU or S/PDIF but for video instead of audio. It has been around a long time so a lot of gear with SDI capability is SD but the SDI protocol officially supports up to 2k or something now.
Short version, SD is (sort of) the resolution of the video signal, SDI is (sort of) a type of digital video connection.