Welcome June
You'll find reasonably priced hydraulic brake sets on Amazon, Ebay, and AliExpress.
Just do a search for Shimano Hydraulic Brakes and you'll find sets like Shimano's MT200 for about £75 and equivalent sets from Clarks for about £60 - sometimes a bit cheaper.
These come ready filled with mineral oil: Generally, the hose lengths are quite generous. On a short/small ebike like the Onebot S6, you may need to shorten the hose(s) for which you'd also need to buy a cut and bleed kit and mineral oil to refill the system. Ditto if the hoses need to be routed through the bike frame internally.
The other thing you need to consider is that mechanical brakes on an ebike usually have safety cut-outs to cut power to the motor on the brake levers. You generally don't get that with hydraulic brake levers unless you add them.
There is a lower-cost alternative that also avoids the fitting issues above which I'm currently trying on my own Fiido 20" folding ebike. You can read about my exploits
here.
Why is it that you feel you want to change your existing brakes?