In most cases you won't be able to make inbound connection when using Mobile Data because of CGNAT and firewalls at ISP level, as explained by acejavelin in comment. See this answer to know the problem in detail and how it can be worked around.
However if you can reach your phone from internet, extending this access to a connected host (PC in your case) is not a problem. Port forwarding is done with routers and when being used as a Wi-Fi hotspot, your phone is in fact acting as a router. In order to forward port, you need to do Destination Address Translation (DNAT) using Linux kernel's builtin firewall userspace tool
First make sure that kernel is configured to forward internet traffic (already set when hotspot is ON):
~# echo 1 >/proc/sys/net/ipv4/ip_forward
Let's say the IP address assigned by hotspot to PC is 192.168.43.10 and you want to forward SSH default port (22):
~# iptables -t nat -I PREROUTING -p tcp --dport 22 -j DNAT --to 192.168.43.10
This is the simplest form of Port Forwarding.
If default POLICY of FORWARD chain in FILTER table is not ACCEPT, define rules explicitly to accept the forwarded packets:
~# iptables -I FORWARD -p tcp -d 192.168.43.10 --dport 22 -j ACCEPT
Also accept outbound forwarded traffic if needed.
And yes, executing all above commands need root access.