To access the Global Search feature you must need to give the below access rights to the Users. Go to Settings -> Users -> Can Access All in One Search.

Once you give the 'Can Access All in One Search' group access to the user after the user can able to access the Global search feature.

You can see in the below screenshot the Global Search or All in one Search Icon will be added on the top-bar section of odoo. So it can be accessed from any screen of the odoo system.

In the Input section, you can type your string, once you add your string in the input box then you need to hit the Enter button so the system will automatically search for matching records of that string.

In the below screenshot, you can see the result of the Search. here multiple sections where the first highlighted one represents Model/Table Name, Second one represents the record name which matched with the searched string. There are multiple records so you can also scroll to check other matching records.

In below the screenshot, you can see that If there are multiple records they will combine in the limit, and if you want to see other records you can click on Show All or Show More to see other records.

To restrict the Models or tables from the Search feature you need to Exclude that Object. Go to Settings -> Users -> Exclude Objects.

The exclude Objects Feature is if you don't want to search from a few tables or objects then you can also ignore those tables during searching.

In below the screenshot, you can see we have added a few Objects or Models. This Object or Model will not be considered during the search. You can add your models according to your needs by clicking on the create button.