You certainly need a junction box, unless one is built in to the fixture.

Yes, you'll stuff the light's "innards" into the ceiling junction box. Example Nicor DSEx. Depending on the fixture and depth of the junction box, this can be difficult. I installed two similar fixtures and it was rather hard because the junction boxes were shallow. I'd buy ONE and make sure you can install it before buying the other three.

Yes, it is possible to replace an older glass globe flush mount light with a new flush mount LED light without installing a new housing. However, it is important to ensure that the wiring and mounting hardware are compatible with the new LED light.

Additionally, I would encourage you to consider upgrading to the latest smart recessed lighting technology. These lights are very convenient and offer advanced features such as remote control, voice control, and integration with smart home systems. They can also be controlled using mobile apps and voice assistants, making them a great choice for those looking to add more convenience and intelligence to their home lighting system.

Ignore the comment by @Krista Deng. There's no need for any of that smart lighting stuff in what appears to be a carport.