[Samsung Galaxy S22 Ultra] TheMystic:
Stock Android and OEMs with near Stock Android implementations do not provide a native firewall that allows users to block internet access completely to apps, i.e block internet over both WiFi and Mobile Data. In such cases, 3rd party apps like Netguard, Karma Firewall, etc. are the only solution.

Most Chinese smartphone brands like OnePlus, iQOO, Oppo, Vivo, Xiaomi, etc. do provide a system managed built-in Firewall. Samsung too has a half-baked implementation that can be put into use effectively.

Check this setting in OneUI:

View attachment 5892833

You'll find this setting under Settings/ Connections/ Data Usage/ Allowed network for apps.

Users who use mobile data exclusively can block apps from connecting to the internet by choosing the 'WiFi only' option for such apps.

But users who use both WiFi and Mobile Data, who are the majority, can also use a simple workaround to block internet access to apps completely. This is where the Data Saver feature of Android comes in handy:

1. Choose Mobile Data only for apps that you want to block from accessing the internet.

2. Under Settings/ Connections/ Data Usage/ Data Saver/ Allowed to use Data when Data Saver is ON, enable the setting only for apps that you want to allow internet access under both WiFi and Mobile Data.

By default, most apps are not allowed to use data when Data Saver is ON (usual exceptions are Google Play Services, Download Manager, Carrier Services, and only a couple of others).

Now if you keep Data Saver ON 24x7, only apps that are allowed to use data while Data Saver is ON will be able to connect to internet. All other apps will be blocked from connecting to the internet.

With this configuration, Data Saver feature will not work for the original intended use (i.e. saving internet data as quite a few apps are now being allowed to access internet even when it is ON), but that should be fine for most users as Mobile Data plans today are very cheap in most regions. You should also consider disabling 'Allow background data usage' too for apps that you don't want connecting to internet.

But if user wants to save data under specific scenarios, then there is always the option to use 3rd party firewall in such cases to limit data use. Netguard has a Lockdown feature which can be used as an additional profile.

ADDITIONAL BENEFITS

1. Allows you to use a real VPN: If you're using a 3rd party firewall (that works by creating a local VPN server) and need to use a real VPN, this method would be extremely handy, because it allows you to use a real VPN whilst still blocking apps you choose from connecting to the internet. If you don't use this method, then those apps will be able to connect to internet during the time you're using a real VPN.

2. Battery savings: Since apps are now being blocked by the system itself (as opposed to a VPN based firewall) from connecting to the internet, there will be some battery savings.

3. No ads: Apps that are blocked from connecting to the internet will not be able to show ads. This too will contribute to additional battery savings.

4. Additional battery savings on WiFi: If your device is connected to the WiFi most of the time, you can expect even more battery savings, especially idle (standby) battery drain.

Hope this helps.