The biggest difference is just in the collocations. In food, you would refer to meat as “tender” but a cookie would be “soft”. If you say you have “tender” muscles, it means it’s painful to touch them. “Soft” muscles means you need to go to the gym! If a person is “tender”, it means they’re thoughtful and sweet. Calling someone “soft” is usually an insult—it means they’re not strong or tough!