How to update Magento core which support different PHP versions and third party modules

Our website is on Magento 2.4.2 and it runs on PHP 7.4. We also have different third party modules running with older versions.

Anyone please help how to update to Magento 2.4.5 p1 which supports PHP 8.1 (is this mean which doesn’t support PHP 7.4?) but the server is currently on PHP 7.4. If I update to PHP 8.1, the website became unavailable due to Magento 2.4.2 running on PHP 8.1. So on which stage of update progress, we have to upgrade PHP to 8.1?

In short, I like to know following:

1. In which stage of update progress upgrade PHP to 8.1 as Magento 2.4.2 not working on PHP 8.1 but latest Magento need PHP 8.1?
2. Do I have to update third party modules first or Magento core first?

Please advice.

Many thanks in advance.