Both mechanical relays, and solid state relays both have their particular characteristics. They both have their particular requirements, and may not necessarily be directly able to be substituted for each other.
Solid state relays however, are extremely fast switching with their speed of response. The circuit design for one or the other, has to be specific.
Most any transistor can work as a switch. There are however, transistors that are designed specificaly for swithing applications.
Most of the time, solid state relays are opticaly isolated. There are however some types that do not use optical design for their isolation. There are also some types that may not be isolated for various reasons.
Web page from a distributor for solid state relays:
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Examples of spec sheets:
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Jerry G. ======