Are these lights bright enough to light a patio?

No. They are decorative accent lights, so expect a warm glow along a railing or pergola rather than usable light for dining or reading.

Can the string be left outside in rain?

It is sold for outdoor lighting, but keep the plug and any connection points sheltered or sealed. Water in the connectors is the usual cause of failure.

Can I connect several sets end to end?

Check the plug and socket on the set you receive before linking strings. Many decorative LED sets are not designed to be joined, and overloading one outlet is a risk.

Do the bulbs need replacing?

LEDs are built to last far longer than incandescent bulbs and usually are not user-replaceable. If a section goes dark, the fault is more often the wiring or a connection than a single bulb.

How is this different from a smart light string?

It has no app, voice control, dimming or color changing. You plug it in and it glows. That makes it simpler and cheaper to own, but less flexible than smart sets.