You can watch them on the following platforms:
On browser
・pixiv Sketch
https://sketch.pixiv.net/lives
On the app
・pixiv Sketch
iOS: https://apps.apple.com/jp/app/pixiv-sketch/id991334925
Android: https://play.google.com/store/apps/details?id=jp.pxv.android.sketch
・pixiv
iOS: https://apps.apple.com/jp/app/pixiv/id337248563
Android: https://play.google.com/store/apps/details?id=jp.pxv.android