The lab creates the user limited (password Lab-Pass-1). Your hand-made cgroup in task 2 lives only until the next reboot, so check before you reboot.

Tasks

With pam_limits, give limited a soft limit of 100 open files, a hard limit of 200 open files, and a hard limit of 50 processes, in a file in /etc/security/limits.d/. Check them from a login shell as limited. Try ulimit and prlimit on your own shell and commands too.
Make a cgroup /sys/fs/cgroup/lab by hand with a memory limit of 50M, no swap, and a limit of 5 tasks. Run Python inside it so that both limits bite: an allocation that the OOM killer stops, and forks that are refused. Leave the cgroup in place, with its counters, for the check.
Give a slice of your own, lab.slice, TasksMax=20 and MemoryHigh=100M with systemctl set-property, run a transient service in it, and read the settings back. Try MemoryMax on a scope and CPUQuota on a service with systemd-run as well.

Hints

A line is user type item value, e.g. limited soft nofile 100; sudo -i -u limited bash -c 'ulimit -Sn; ulimit -Hn; ulimit -Hu'. ulimit -a, prlimit --nofile=8:8 COMMAND, /proc/PID/limits.
mkdir /sys/fs/cgroup/lab, then write memory.max, memory.swap.max and pids.max; move a shell into it with echo $$ > cgroup.procs. python3 -c "x = bytearray(200 * 1024 * 1024)"; a loop of os.fork(). memory.events, pids.events.
systemctl set-property lab.slice ..., systemd-run --unit=NAME --slice=lab.slice sleep 60, systemctl show lab.slice -p TasksMax -p MemoryHigh; systemd-run --scope -p MemoryMax=50M, -p CPUQuota=20%; systemd-cgls, systemd-cgtop.