The solids are supposed to stay in the tank, where they can be pumped out and disposed of as septage. If the solids get into the drain field, they will plug it solid and you will end up installing a whole new system. Adding bacteria won't hurt anything, but it won't help either.
Avoid grease and oils, which do not digest, and form the heavy crust on top of the liquid.
It's critical that the baffles are in good shape. The inlet baffles kill the momentum of sewage entering the tank, while the exit baffles skim the grease off the top and only allow liquids into the drain field.
How long your tank can go between pumping depends on what you put into it. Some plants have a high silica content, and leave a lot of sludge. Food scraps don't digest well, though there are garbage disposals with enzyme reservoirs designed to help that along. Smaller tanks or tanks with heavier usage will need to be pumped more often.
Avoid using a septic tank as an RV dump station. The RV chemicals will harm it, and the massive rush of water will overwhelm the baffles. For a similar reason, low flush toilets and front loading washing machines are a good idea. The less water you push through a septic tank, the longer the solids get to settle out.
Some people will brag about how long they have gone without pumping their septic tank. That works fine until it doesn't, and they end up buying a whole new system.