Thanks! There is no one size fits all unfortunately for sharing across social media, but it is getting a little easier and more universal as channels/platforms are starting to accept more sizes. I think what you are referring to with text on top and bottom are typically squares.
Start with a 1080 x 1080 px square box.
Drop a wide video (1920 x 1080 px) in the center of the square.
You will now have blank space on the top and bottoms to add text and you could add color blocks/shapes if you want to spruce up the background.
Square works in a lot of places now, so maybe start there and experiment with how it looks on the different platforms and adjust as needed.
Let me know if you need help with anything.