Try the Commands Troubleshooting and Help page if you get stuff with server errors.
New Spawn Eggs in Minecraft 1.17 java
Minecraft Versions Permalinks
If you find yourself using a particular version all the time, you can link directly. There is some crossover between versions, so there will be quirks.
Detailed Mob Generators
Generators with pattern selectors
Other Mobs NBT Tag Data
Summon mob page with notes about custom mob options.