Generally, if you're looking at two lenses as different as this, you may not be quite ready to purchase a second lens, yet. Ask yourself what it is you want the most that your 16-50mm kit lens cannot do. Then figure out which lens is going to give you that. To me, someone's closer to being ready to buy a new lens when they're trying to decide between two lenses of the same type.

In general, a longer telephoto zoom lens (like the 55-210mm) will give you more "reach" to shoot subjects that are farther away; a fast prime (like the 50mm f/1.8) will give you much larger aperture settings to work with.

Each lens is defined by two main specs: focal length (in mm) and maximum aperture (the largest aperture setting you can use on the lens).

With focal lengths, the bigger the number is, the more you're "zoomed in" (narrower the view); the smaller the number is, the more you're "zoomed out" (wider the view).

With max. apertures, the smaller the f-number, the bigger the aperture opening is. And on a zoom lens, if two f-numbers are give, those are the maximum apertures at either end of the zoom range. So with the 55-210, it can open up to f/4.5 at the 55mm end of the range, and only to f/6.3 at the 210mm end of the range. f/4 is "middling", f/2.8 or wider is "fast" and f/5.6 or smaller is "slow" in terms of the shutter speeds you need to get good exposures while handholding.

IOW, the 50/1.8 can handle lower light situations much better than the 55-210mm can.

If I go with the zoom lens can I shoot a decent portrait?

Yes. But you may need to add lighting with something like a flash to do so, and you may need to work from farther away because of the longer focal lengths. You may still be able to blur backgrounds, but probably not as well as the f/1.8 max. aperture of the 50mm lens can. A professional photographer would prefer an f/2.8 70-200 telephoto zoom for portraits.

Will I get the minimum focal range of F4.5 MM while maximum zoom?

Assuming you mean "maximum aperture" of f/4.5 while zoomed to 210mm? No. The widest the lens can open up at 210mm is f/6.3, as its name says. Not all lenses have a variable max. aperture, but lower cost budget zoom lenses nearly always do.

This is because the f-number is a ratio of the aperture opening diameter divided by the focal length:

opening_diameter / focal_length

In these lower cost lenses, the opening just stays the same size, so as the focal length increases, the f-number gets smaller.

If I choose the prime lens, will this use the same focal length F1.8 on maximum zoom range?

A prime does does not zoom at all. It can only do 50mm. If you want to change the framing, you have to change your vantage point. You can always use f/1.8 because of this. But it's not always the best option to shoot with a lens "wide open" (at max. aperture) because that's typically where a lens will be at its softest, and the most prone to vignetting and chromatic aberration. "Stopping down" (using a smaller aperture setting) can often make a lens look sharper. It can also increase the depth of field (how much of the scene is in focus from front-to-back) so that it's harder to misfocus.

I heard when you zoom it changes the aperture which might affect image/video quality and make it dark.

Yes. With a variable max. aperture zoom lens, the aperture will get smaller the more you zoom in.

Can anyone help me choose the right lens?

That's very difficult to do when we don't know what you want to use the lens for. But in general, interchangeable lenses are best as special purpose tools and not as general-purpose ones. The basic answer if you want telephoto reach and you want a fast portrait prime at budget prices is to get both lenses. A professional-grade 70-200mm f/2.8 lens that can do both tends to be very expensive.

I want a decent zoom (not exactly wildlife shoot where I get closer to Lion!!) and a good portrait lens.

Arguably, that's why Sony bundled a 16-50mm f/3.5-5.6 kit lens into the box with your ZV-E10. It's a decent zoom lens that covers most of the walkaround range (roughly equivalent to the 1x-3x zoom range of a smartphone camera), and at the 50mm end, with good light, it can be a decent portrait lens as well. It's a decent consumer-grade all-rounder. It is slow and only wide-to-barely-telephoto, but it can cover landscapes, street, and portraits. There are better lenses for these subjects, but they're more expensive than the lens you already own.

Early on, most folks fall into the pattern of thinking they just need a better camera to take better pictures. Then, reading on online forums how a kit lens sucks and because the camera didn't magically make all their photos pro-quality, they figure it has to be the lens. And both the camera and lens do matter in defining what/how you can take pictures.

But the quality of your photos may rely even more heavily on your ability to compose, to control exposure, to focus the lens, your timing, ability to light or find good light, post-processing skills, and visual imagination. And that's all on you, not the gear. You may want to consider investing some time and money in learning materials like books, videos, classes, or workshops (or just more time spent shooting) while you learn how to figure out what lens you want to buy next the most.