This website hosts a large volume of material dedicated to teaching ultrasound - from the basics of sonography, through the functioning of a machine, to clinical applications of ultrasound including POCUS in an emergency setting.
These websites contain plentiful freely accessible material pitched at undergraduate level.
'Radiology Basics' covers basic cross-sectional imaging and was produced by a former Leicester medical student, Melisa Sia, with consultant radiologist Dr Vikas Shah. It is available as a free iBook download or on the Radiology Cafe website.
Searching #FOAMrad on any social media platform will bring up a lot of educational material. Here are some suggested accounts to follow: