Having a paid edition of Quartus Prime Standard, is another license required for ModelSim? If so, what are features in paid ModelSim over Free version?

Yes Modelsim is other product with other license.

see https://www.intel.com/content/www/us/en/software/programmable/quartus-prime/model-sim.html for more details.

Best Regards,

Johi.

The ModelSim-Intel FPGA Starter edition is included with a license. If you want to use the full ModelSim-Intel FPGA edition, it is an additional license.