Fitting Shimano pedals into a 3/4" crank socket
I've never heard of a 3/4&quot; thread on a pedal on a bicycle.  It sounds like they intend for you to buy pedals from the maker of your exercise bike.
However if you are handy, it may be possible to make a pair of adapter bushings from metal.

The pedal threads will be 9/16&quot; with 20 TPI (which is not the more common 18TPI)   Sadly 20TPI is unique to bicycle pedals, so you're not going to be able to use plumbing parts for this.
One has to be left-hand thread for the left pedal's shaft
And the outside will have to match the 3/4&quot; and whatever thread pitch is in your bike's cranks.  This may be 16 TPI for UNF thread, or 10 TPI for UNC, or something weirder.
Also its not guaranteed that your 3/4&quot; threads are opposite-handed.

Given your bike is Chinese, its probably metric for the most part, so there's a chance your pedals have an M18 or an M20 thread, 3/4&quot; is about 19mm so right in the middle of those options.   None of them are pedal threads I've heard of though.
This would be a simple job for an engineering firm to fab up from a couple of small disks of steel or maybe aluminium.   It would help if you could give them both cranks and both pedals for test-fitting too.

There was a short lived thread of 1 inch, called Shimano Dyna drive.  Adapters from that down to 9/16&quot; look like this.  Notice the inner is completely within the outer, because there's enough metal to make this possible.

The other option is to stagger them a little as in this 9/16 to 1/2 inch adapter set.  Downside of these is they increase your &quot;q-factor&quot; which means your pedals are further apart.

Images are for description of the adaptor required, not a specific product recommendation.  Pictured products have the wrong threads for OP's need.