Pick a date and time and copy the code into your message. Everyone sees the time in their own time zone.
A timestamp code like <t:1791763200:F> contains a moment in time as a Unix timestamp and a letter for the format. When you send it in a message, Discord shows the date and time in the time zone and language of each reader. This is useful for events with people from different countries.
Use the long date and time for events, and the relative format for deadlines, like "in 3 days". You can put several timestamps in one message, for example the date and the relative time next to each other.
That's how timestamps work. Discord shows each reader the time in their own time zone, so everyone sees the right local time for the same moment.
Yes. A bot can put the same codes in its messages and in the description and fields of embeds.
Yes. A relative timestamp like "in 2 hours" counts down while the message is open and changes to "2 hours ago" afterwards.

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