Did you know there is a non-destructively method you can use to make symmetrical objects? Well, this is possible thanks to the Mirror Modifier, do you want to know how you can apply it and use it? Keep reading!
Basic application
The Mirror Modifier is used to duplicate and flip the direction of all the points you edit in your mesh across the X, Y, and Z axes. To demonstrate this, you’ll use the primitive model of the monkey offered by Blender. Just press Shift + A, select Mesh Option, and pick the Monkey, then press Tab on your keyboard to go to the Edit mode.
Once you’re there, you must delete half of your model so the modifier can do its job correctly and Blender will jot duplicate the vertices of both sides. Therefore, place your model in the middle of an axis, go into the Wireframe View, select only the vertices of one side, click X to bring up the delete menu, and select the option of delete vertices. Now, you have a model perfectly divided in half.

Excellent! now go to the Modifier Tag on the right side of your screen, click on Add Modifier, search in the Generate category the Mirror option, and behold a complete and perfect symmetrical model!

Note: If you have a model and don’t erase the half, then when you add the Mirror Modifier, you will have double or even triple vertices very close to each other. That is because the two faces will reflect in their opposite flank (the left one will reflect on the right side and vice versa).
Settings
On the right side of your screen, you will see some options, but let’s focus on Bisect and Flip, which have letters corresponding to the axes. The Bisect will essentially be like the base operation, which means it will automatically select half of your model according to your chosen axis.
Note: Usually, Blender will select the right as the reference side.
But if you want the left side as the primary reference, you can use the Flip option to set and change the main flank, just remember to choose the axis you need and the operation will do the rest. So, be confident and explore what you think fits you better.
Final recommendations
Finally, Blender will reflect your model according to where the pivot point is. So, be careful because if you have a gap between the pivot point and the initial part of your model, Blender will reflect that space too.

to attach the nearest vertices along the centerline and avoid any gap, go to the Mirror Modifier window at your right and click on the Clipping Option.

All it takes is a bit of curiosity to explore all the options you have with this modifier. Good Luck!