extend How to only affect the current

How does the set extension only affect the current, not the overall situation
```
const config = {
thresholds: [
{ l: 's', r: 44, d: S },
{ l: 'm', r: 89 },
{ l: 'mm', r: 44, d: MIN },
{ l: 'h', r: 89 },
{ l: 'hh', r: 21 * 60, d: MIN },
{ l: 'd', r: 1 },
{ l: 'dd', r: Infinity, d: H },
],
};
dayjs.extend(relativeTime, config)
dayjs.extend(updateLocale);
dayjs.updateLocale('en', {
relativeTime: {
future: 'in %s',
past: '%s ago',
s: 'a few seconds',
m: 'a minute',
mm: '%d minutes',
h: 'an hour',
hh: (number: number) => {
return getHourRelative(number, true)
},
d: 'a day',
dd: (number: number) => {
return getDayRelative(number, true)
},
M: 'a month',
MM: '%d months',
y: 'a year',
yy: '%d years'
}
})
```

Clarify whether the relativeTime configuration passed to dayjs.extend should affect only the current use or the overall library, and define the intended scope of updateLocale('en'). Reproduce the provided example and compare relative-time output before and after the extension; completion requires an agreed behavior and tests for that scope.

Written by the indexing model from the issue text.