Most seeds will germinate between 60-80 degrees. If it is warmer outside then a heat mat is not needed. Some seeds like tomatoes and cool crops can germinate at 50 degrees. No matter where you are there is a growing season. Not much will grow in 100 degree weather unless you live in the tropics or the desert and grow climate adapted plants. If you live in a tropical zone, not a temperate zone, it will be more challenging to grow temperate plants. You will need to choose cultivars for heat and disease tolerance. You may also need to find ways like mulching, adding organic matter, and drip irrigation to slow water loss. Some crops that like full sun in other climates, may benefit from shading either from shade cloth (which increases heat and humidity), and choosing a planting site that only gets morning sun.
In warm areas, there is no need to start seeds indoors. You can start your seeds directly in the ground if the ground temperature is above 60 degrees or I start some seeds in pots on an outdoor bench in the sun. No hardening off required, lots of air and heat is supplied by the sun. Shade cloth on the seedling bench is optional.
The middle of summer is my tropical "winter garden time". I basically don't plant much above 90 degrees. I only harvest, solarize, or if I am composting, I will do trench composting at that time and cover the unused beds with tarps to keep the weeds down. It takes too much water and effort to grow annuals in the heat of summer. I just take care of the perennial crops that do fine with the heat like the hot peppers, eggplant, sweet potato, okra, long beans, banana, papaya, and the other tropical crops that are adapted to the climate. Some of the Florida tomatoes can grow in 100 degree heat, but they require a lot of care in summer to keep producing. I can keep tomatoes going during summer with water and pest management, but most of them will stop producing fruit and flowers until the weather is milder. Plants know when it is time to grow. Seeds will not germinate unless conditions are good and plants will not flower and fruit unless the plant is mature and healthy enough to support fruit and flowers.