Very very simplified: Basically, you have a blob of fat that the mRNA sits in. The fat sticks to the cell, merges with the cell membrane and the mRNA pokes into the cell. It's not a modified virus, it's a much simpler structure but it has a similar mechanism. It's much simpler in that it can't replicate itself or do much else. It's literally just the code for a protein that's found on the surface of the covid virus, in a blob of fat.

That's for the BioNTech/Pfizer vaccine. Other vaccines, such as the Oxford/AstraZeneca one do use a modified virus.