Such as with prepaid data sim cards, prepaid data eSIM plans network types will depend on multiple factors :
The type of plan you purchased. Some eSIM data plans are sold as 3G only. This means that your eSIM provider will throttle your type of network (and speed, therefore) to 3G. Be very cautious about that before buying a data eSIM plan, although most plans include 4G. You should also make sure that your plan supports 5G in case you have a 5G compatible device and you expect this type of speeds.
The commercial agreements your eSIM provider has with local operators. Most prepaid eSIM providers (Airalo, Ubigi, Redteago, etc) are not local operators. They are what the industry call "MVNOs" (mobile virtual network operators). In other terms, they have agreements with multiple local operators (Vodafone, AT&T, Orange, Airtel, etc) in multiple countries to be able to use their network. So for instance, if you purchase a North America plan from an eSIM provider and end up connected to the AT&T network while travelling in the USA, this is because your provider has signed roaming agreement with (and "buys its data") from AT&T.
This is usually a big benefit for users, as MVNOs usually have agreements with multiple local operators in the same country, which will allow you to switch network and search for the best connectivity.
On some rare occasions, some eSIM providers will have agreements with local providers that only include 3G speed.
Your location (local infrastructure). Quite obviously, your location & the infrastructure you are connected to will be the ultimate criteria that determines which network you will get. Do not expect 100MB/s speeds if you have poor connectivity in the middle of nowhere !